Debt - Estimated Fair Value of Our Total Debt (Parenthetical) (Detail)	1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 31, 2015	Mar. 31, 2015	Dec. 31, 2015
3.45% Senior Notes due August 2015 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	3.45%		3.45%
Senior notes, maturity date	2015		2015
3.05% Senior Notes due March 2016 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			3.05%
Senior notes, maturity date			2016
2.50% Senior Notes due March 2017 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			2.50%
Senior notes, maturity date			2017
4.00% Senior Notes due March 2018 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes		4.00%	4.00%
Senior notes, maturity date		2018	2018
7.50% Senior Notes due March 2019 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			7.50%
Senior notes, maturity date			2019
4.90% Senior Notes due August 2020 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			4.90%
Senior notes, maturity date			2020
4.625% Senior Notes due March 2021 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			4.625%
Senior notes, maturity date			2021
3.95% Senior Notes due March 2022 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			3.95%
Senior notes, maturity date			2022
5.95% Senior Notes due April 2025 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes		5.95%	5.95%
Senior notes, maturity date		2025	2025
6.20% Senior Notes due August 2040 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			6.20%
Senior notes, maturity date			2040
6.05% Senior Notes due March 2041 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			6.05%
Senior notes, maturity date			2041
5.25% Senior Notes due March 2042 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			5.25%
Senior notes, maturity date			2042
6.95% Senior Notes due April 2045 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes		6.95%	6.95%
Senior notes, maturity date		2045	2045