Credit Facility - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended	1 Months Ended	3 Months Ended
	Sep. 30, 2014	Oct. 31, 2013	Sep. 30, 2014
Line of Credit Facility [Line Items]
Interest on borrowings, description			Either (a) a Eurodollar Rate formula or (b) the Base Rate formula, each as set forth in the Credit Agreement.
Five-year Unsecured Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility available amount		$ 650
Credit Agreement term		5 years
Letters of credit sublimit under credit facility		100
Maximum amount of credit facility		$ 900
Credit Agreement due date			Oct. 23, 2018
Commitment fee percentage on undrawn balance	0.25%
Five-year Unsecured Revolving Credit Facility [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Applicable margin for borrowing	0.50%
Five-year Unsecured Revolving Credit Facility [Member] | Eurodollar [Member]
Line of Credit Facility [Line Items]
Applicable margin for borrowing	1.50%
Five-year Unsecured Revolving Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Operating income leverage ratio		3.0
Five-year Unsecured Revolving Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Interest coverage ratio		3.0