Senior Notes - Summary of Outstanding Senior Notes (Parenthetical) (Detail)	Sep. 30, 2017	Aug. 31, 2017	Dec. 31, 2016
3.45% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Interest rate of senior notes	3.45%		3.45%
7.875% Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Interest rate of senior notes	7.875%	7.875%	7.875%
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%