Credit Agreement, Commercial Paper and Senior Notes - Summary of Carrying Value of Senior Notes, Net of Unamortized Discount and Debt Issuance Costs (Parenthetical) (Detail)	Dec. 31, 2016	Dec. 31, 2015
5.875% Senior Notes due 2019 [Member]
Debt Instrument [Line Items]
Interest rate of senior notes	5.875%	5.875%
3.45% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Interest rate of senior notes	3.45%	3.45%
5.70% Senior Notes due 2039 [Member]
Debt Instrument [Line Items]
Interest rate of senior notes	5.70%	5.70%
4.875% Senior Notes due 2043 [Member]
Debt Instrument [Line Items]
Interest rate of senior notes	4.875%	4.875%