Quarterly report pursuant to Section 13 or 15(d)

Long-Term Debt (Details)

v3.8.0.1
Long-Term Debt (Details)
9 Months Ended 12 Months Ended
Dec. 31, 2020
Oct. 30, 2017
USD ($)
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Mar. 31, 2019
Apr. 01, 2018
Mar. 31, 2017
USD ($)
Debt Instrument [Line Items]                    
Less current portion           $ (1,875,000)       $ (20,250,000)
Long-term debt, net of current maturities and deferred debt issuance costs and debt discounts of $9,089 and $524 as of December 31, 2017 and March 31, 2017, respectively           239,036,000       60,226,000
Debt issuance costs, net           9,089,000       524,000
Repayments of long-term debt           91,000,000 $ 10,125,000      
Maximum leverage ratio to secure additional borrowing   4.00                
Loans Payable                    
Debt Instrument [Line Items]                    
Debt Instrument, face amount                   5,905,000
Revolving credit facility                    
Debt Instrument [Line Items]                    
Outstanding borrowings           $ 0        
Interest rate at period end (as a percent)           5.13%        
Capacity available under credit facility           $ 55,085,000        
Through March 31, 2019 | Loans Payable                    
Debt Instrument [Line Items]                    
Repayments of notes payable           625,000        
Due in April 2019 | Loans Payable                    
Debt Instrument [Line Items]                    
Repayments of notes payable           233,750,000        
Revolving credit facility                    
Debt Instrument [Line Items]                    
Outstanding borrowings           0       0
Revolving credit facility | Line of Credit                    
Debt Instrument [Line Items]                    
Debt Instrument, face amount   $ 60,000,000                
Repayments of lines of credit   $ 6,000,000                
Line of credit facility, commitment fee percentage   0.50%                
Debt covenant, coverage ratio   1.25                
Revolving credit facility | Base Rate | Line of Credit                    
Debt Instrument [Line Items]                    
Basis spread on variable rate   2.25%                
Revolving credit facility | London Interbank Offered Rate (LIBOR) | Line of Credit                    
Debt Instrument [Line Items]                    
Basis spread on variable rate   3.25%                
Revolving credit facility | Canadian Base Rate [Member] | Line of Credit                    
Debt Instrument [Line Items]                    
Basis spread on variable rate   2.25%                
Revolving credit facility | CDOR | Line of Credit                    
Debt Instrument [Line Items]                    
Basis spread on variable rate   3.25%                
Variable Rate Term Loan due October 2024 | Loans Payable                    
Debt Instrument [Line Items]                    
Variable Rate Term Loans           240,911,000       0
Debt issuance costs, net           9,089,000        
Variable Rate Term Loan due April 2019 | Loans Payable                    
Debt Instrument [Line Items]                    
Variable Rate Term Loans           0       $ 80,476,000
Debt issuance costs, net           $ 524,000        
Variable Rate Senior Secured Term Loan B | Secured Debt                    
Debt Instrument [Line Items]                    
Debt Instrument, face amount   $ 250,000,000                
Variable interest rate floor   1.00%                
Prepayment premium   1.00%                
Additional borrowing capacity   $ 30,000,000                
Variable Rate Senior Secured Term Loan B | Secured Debt | Subsidiary Stock                    
Debt Instrument [Line Items]                    
Guarantor obligations, percentage   100.00%                
Variable Rate Senior Secured Term Loan B | Secured Debt | Stock of First Tier Material Foreign Subsidiaries, US Borrower, and US Subsidiary                    
Debt Instrument [Line Items]                    
Guarantor obligations, percentage   65.00%                
Variable Rate Senior Secured Term Loan B | Base Rate | Secured Debt                    
Debt Instrument [Line Items]                    
Basis spread on variable rate   2.75%                
Variable Rate Senior Secured Term Loan B | London Interbank Offered Rate (LIBOR) | Secured Debt                    
Debt Instrument [Line Items]                    
Basis spread on variable rate   3.75%                
Term Loan A due April 2019                    
Debt Instrument [Line Items]                    
Repayments of long-term debt   $ 70,875,000                
Thermon Heating Systems Inc.                    
Debt Instrument [Line Items]                    
Debt proceeds used as consideration for business acquisition   $ 164,900,000                
Scenario, Forecast | Revolving credit facility | Line of Credit                    
Debt Instrument [Line Items]                    
Maximum leverage ratio 3.75   4.50 5.00 5.50          
Scenario, Forecast | Variable Rate Senior Secured Term Loan B | Secured Debt                    
Debt Instrument [Line Items]                    
Quarterly amortization of debt, percent                 0.25%  
Repayment requirement, percentage of excess cash flow               50.00%    
Reduced repayment requirement, percentage of excess cash flow               25.00%    
Maximum leverage ratio requirement for reduced repayment               4.00    
Minimum leverage ratio requirement for reduced repayment               3.50    
Minimum repayment requirement, percentage of excess cash flow               0.00%    
Maximum leverage ratio requirement for minimum repayment               3.50