Credit Facilities - Outstanding Amount of Credit Facilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Total debt	$ 625,000	$ 765,466
Less debt due within one year	375,000	265,466
Long-term debt, excluding current portion	250,000	500,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Unsecured revolving line of credit, $1,200,000,000, LIBOR plus 0.75% variable	125,000	265,466
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	250,000
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	$ 250,000