Credit Facilities - Outstanding Amount of Credit Facilities (Parenthetical) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013
Line of Credit Facility [Line Items]
Unsecured revolving line of credit	$ 850,000,000
Debt instrument basis spread on variable rate	0.75%
4.67% Series C Senior Unsecured Notes [Member]
Line of Credit Facility [Line Items]
Face value of Senior Unsecured Notes	250,000,000,000
Interest rate of Senior Unsecured Notes	4.67%
Due date of Senior Unsecured Notes	Nov. 30, 2013
November 30, 2011, Series D and E Senior Unsecured Notes, $250,000,000, 3.35% Fixed, Due November 30, 2016 [Member]
Line of Credit Facility [Line Items]
Face value of Senior Unsecured Notes	250,000,000,000
Interest rate of Senior Unsecured Notes	3.35%
Due date of Senior Unsecured Notes	Nov. 30, 2016
December 2, 2013, Series F Unsecured Notes, $250,000,000, 2.99% Fixed, Due December 2, 2023 [Member]
Line of Credit Facility [Line Items]
Face value of Senior Unsecured Notes	$ 250,000,000
Interest rate of Senior Unsecured Notes	2.99%
Due date of Senior Unsecured Notes	Dec. 02, 2023