Credit Agreement, Commercial Paper and Senior Notes - Summary of Carrying Value of Senior Notes, Net of Unamortized Discount and Debt Issuance Costs (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Senior Notes	$ 1,980,884	$ 1,979,778
5.875% Senior Notes due 2019 [Member]
Debt Instrument [Line Items]
Senior Notes	498,679	498,146
3.45% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Senior Notes	247,879	247,605
5.70% Senior Notes due 2039 [Member]
Debt Instrument [Line Items]
Senior Notes	492,812	492,663
4.875% Senior Notes due 2043 [Member]
Debt Instrument [Line Items]
Senior Notes	$ 741,514	$ 741,364