Credit Agreement and Senior Notes - Aggregate Maturities of Senior Notes (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt Disclosure [Abstract]
2016	$ 0
2017	0
2018	0
2019	500,000
2020	0
Thereafter	1,500,000
Total maturities of senior notes	2,000,000
Less: unamortized discounts	(5,227)
Total maturities of senior notes, net of unamortized discount	$ 1,994,773	$ 2,244,488