Revolving Line of Credit (Detail)	9 Months Ended
	Jun. 30, 2017 USD ($)
Line of Credit Facility [Line Items]
Unsecured revolving line of credit	$ 500,000,000
Option to increase borrowing capacity	$ 100,000,000
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	$ 296,000,000
Interest rate of borrowings outstanding	2.334%
Long-term Debt
Line of Credit Facility [Line Items]
Borrowings outstanding	$ 275,000,000
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%