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Bank Credit Arrangements - Additional Information (Detail) - USD ($)
9 Months Ended
Jun. 30, 2018
Sep. 30, 2017
Mar. 22, 2016
Debt Instrument [Line Items]      
Notional balance $ 60,000,000    
Interest rate swap description (i) notional reduction amounts and dates; (ii) LIBOR settlement rates; (iii) rate reset dates; and (iv) term/maturity.    
Interest rate swap asset $ 1,689,000 $ 815,000  
Bank credit arrangement, fixed interest rate percentage 2.76%    
Credit facility with a commercial bank $ 30,000,000    
Expiration date of credit facility Mar. 31, 2021    
Borrowings outstanding under credit facility $ 0 $ 0  
Cash compensating balance 1,000,000    
Magellan [Member]      
Debt Instrument [Line Items]      
2018 1,125,000    
2019 5,250,000    
2020 6,000,000    
2021 $ 39,000,000    
Five - Year Term Loan [Member] | Magellan [Member]      
Debt Instrument [Line Items]      
Term loan     $ 60,000,000