Annual report pursuant to Section 13 and 15(d)

Long-Term Debt (Details)

v3.8.0.1
Long-Term Debt (Details)
3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2020
Oct. 30, 2017
USD ($)
Mar. 31, 2018
USD ($)
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Mar. 31, 2019
Mar. 31, 2018
USD ($)
Mar. 31, 2017
USD ($)
Mar. 31, 2016
USD ($)
Apr. 01, 2018
Debt Instrument [Line Items]                      
Less current portion     $ (2,500,000)         $ (2,500,000) $ (20,250,000)    
Long-term debt, noncurrent     214,533,000         $ 214,533,000 60,226,000    
Debt instrument, maturity date               Apr. 30, 2019      
Debt issuance costs, net     $ 7,967,000         $ 7,967,000 524,000    
Repayments of Long-term Debt               $ 116,000,000 13,500,000 $ 13,500,000  
Debt Instrument, Debt Covenant, Maximum Leverage Ratio to Secure Additional Borrowing   4.0                  
Revolving Credit Facility [Member]                      
Debt Instrument [Line Items]                      
Interest rate at period end (as a percent)     5.41%         5.41%      
Capacity available under credit facility     $ 55,384,000         $ 55,384,000      
Outstanding borrowings     0         0      
Loans Payable [Member]                      
Debt Instrument [Line Items]                      
Long-term debt     217,033,000         217,033,000 80,476,000    
Period One [Member] | Loans Payable [Member]                      
Debt Instrument [Line Items]                      
Repayments of Notes Payable               625,000      
Period Three [Member] | Loans Payable [Member]                      
Debt Instrument [Line Items]                      
Repayments of Notes Payable     25,000,000                
Revolving Credit Facility [Member]                      
Debt Instrument [Line Items]                      
Outstanding borrowings     $ 0         $ 0 $ 0    
Revolving Credit Facility [Member] | Line of Credit [Member]                      
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 Instrument, Debt Covenant, Coverage Ratio   1.25                  
Variable Rate Senior Secured Term Loan B [Member] | Secured Debt [Member]                      
Debt Instrument [Line Items]                      
Debt instrument, face amount   $ 250,000,000                  
Debt Instrument, Variable Interest Rate Floor   1.00%                  
Debt Instrument, Prepayment Premium   1.00%                  
Line of Credit Facility, Additional Borrowing Capacity   $ 30,000,000                  
Term Loan A due April 2019 [Member]                      
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 [Member] | Revolving Credit Facility [Member] | Line of Credit [Member]                      
Debt Instrument [Line Items]                      
Maximum leverage ratio 3.75     4.5 5.0 5.5          
Scenario, Forecast [Member] | Variable Rate Senior Secured Term Loan B [Member] | Secured Debt [Member]                      
Debt Instrument [Line Items]                      
Debt Instrument, Quarterly Amortization, Percent                     0.25%
Debt Instrument, Debt Covenant, Repayment requirement, Percentage of Excess Cash Flow             50.00%        
Debt Instrument, Debt Covenant, Reduced Repayment Requirement, Percentage of Excess Cash Flow             25.00%        
Debt Instrument, Debt Covenant, Maximum Leverage Ratio Requirement For Reduced Repayment             4.0        
Debt Instrument, Debt Covenant, Minimum Leverage Ratio Requirement For Reduced Repayment             3.5        
Debt Instrument, Debt Covenant, Minimum Repayment Requirement, Percentage of Excess Cash Flow             0.00%        
Debt Instrument, Debt Covenant, Maximum Leverage Ratio Requirement For Minimum Repayment             3.5        
Subsidiary Equity [Member] | Variable Rate Senior Secured Term Loan B [Member] | Secured Debt [Member]                      
Debt Instrument [Line Items]                      
Guarantor Obligations, Liquidation Proceeds, Percentage   100.00%                  
Stock of First Tier Material Foreign Subsidiaries, Domestic Borrower, and Domestic Subsidiary [Member] | Variable Rate Senior Secured Term Loan B [Member] | Secured Debt [Member]                      
Debt Instrument [Line Items]                      
Guarantor Obligations, Liquidation Proceeds, Percentage   65.00%