Revolving Line of Credit - Additional Information (Detail)	12 Months Ended
	Sep. 30, 2015 USD ($)	Nov. 30, 2014 USD ($)
Line of Credit Facility [Line Items]
Unsecured revolving line of credit	$ 400,000,000	$ 200,000,000
Revolving credit facility, expiration date	Dec. 30, 2019
Credit facility restrictive covenant, minimum fixed charge ratio	2.5
Credit facility restrictive covenant, maximum consolidated leverage ratio	3.0
Credit facility restrictive covenant, maximum consolidated leverage ratio step up	3.5
Borrowings outstanding	$ 232,000,000
Interest rate of borrowings outstanding	1.601%
Federal Fund Rate
Line of Credit Facility [Line Items]
Debt instrument basis spread on variable rate	0.50%
LIBOR
Line of Credit Facility [Line Items]
Debt instrument basis spread on variable rate	1.00%
LIBOR | Minimum
Line of Credit Facility [Line Items]
Debt instrument basis spread on variable rate	1.00%
LIBOR | Maximum
Line of Credit Facility [Line Items]
Debt instrument basis spread on variable rate	1.875%
Base Rate | Minimum
Line of Credit Facility [Line Items]
Debt instrument basis spread on variable rate	0.00%
Base Rate | Maximum
Line of Credit Facility [Line Items]
Debt instrument basis spread on variable rate	0.875%
Long-term debt
Line of Credit Facility [Line Items]
Borrowings outstanding	$ 200,000,000