Credit Facility - Additional Information (Detail) - Jun. 30, 2015 - Five-year Unsecured Revolving Credit Facility [Member]	USD ($)	USD ($)
Line of Credit Facility [Line Items]
Unsecured revolving credit facility available amount	$ 650,000,000	$ 650,000,000
Credit Agreement term		5 years
Letters of credit sublimit under credit facility	100,000,000	$ 100,000,000
Maximum amount of credit facility	$ 900,000,000	$ 900,000,000
Credit Agreement due date		Oct. 23, 2018
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%
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%
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