Credit Facility - Additional Information (Detail) - Unsecured Revolving Credit Facility [Member]			1 Months Ended	3 Months Ended
	Oct. 23, 2015	Sep. 30, 2015 USD ($)	Oct. 31, 2015	Sep. 30, 2015 USD ($)
Line of Credit Facility [Line Items]
Unsecured revolving credit facility available amount		$ 650,000,000		$ 650,000,000
Letters of credit sublimit under credit facility		100,000,000		100,000,000
Maximum amount of credit facility		$ 900,000,000		$ 900,000,000
Interest on borrowings, description				Either (a) a Eurodollar Rate formula or (b) the Base Rate formula, each as set forth in the Credit Agreement.
Commitment fee percentage on undrawn balance		0.25%
Amounts drawn under credit facility		$ 0		$ 0
Maximum [Member]
Line of Credit Facility [Line Items]
Operating income leverage ratio				3.0
Minimum [Member]
Line of Credit Facility [Line Items]
Interest coverage ratio				3.0
Base Rate [Member]
Line of Credit Facility [Line Items]
Applicable margin for borrowing		0.50%
Eurodollar [Member]
Line of Credit Facility [Line Items]
Applicable margin for borrowing		1.50%
Subsequent Event [Member]
Line of Credit Facility [Line Items]
Credit Agreement due date			Oct. 23, 2020
Commitment fee percentage on undrawn balance	0.225%