Credit Facilities - Outstanding Amount of Credit Facilities (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Unsecured revolving line of credit, $850,000,000, LIBOR plus 0.75% variable	$ 264,658,000	$ 0
Total debt	764,658,000	500,000,000
Less debt due within one year	264,658,000	250,000,000
Long-term debt, excluding current portion	500,000,000	250,000,000
4.67% Series C Senior Unsecured Notes [Member]
Line of Credit Facility [Line Items]
November 30, 2008, Series C Senior Unsecured Notes, $250,000,000, 4.67% fixed, due November 30, 2013		250,000,000
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]
Senior unsecured notes	250,000,000	250,000,000
December 2, 2013, Series F Unsecured Notes, $250,000,000, 2.99% Fixed, Due December 2, 2023 [Member]
Line of Credit Facility [Line Items]
Senior unsecured notes	$ 250,000,000