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Bank Credit Arrangements - Additional Information (Detail) - USD ($)
9 Months Ended
Jun. 30, 2017
Sep. 30, 2016
Mar. 22, 2016
Oct. 01, 2015
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 $ 795,000      
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]        
Term loan       $ 60,000,000
2017 1,125,000      
2018 4,500,000      
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