Debt - Schedule of Debt (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2014	Mar. 31, 2014
7.375% Senior Notes due 2014 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	7.375%	7.375%
Senior notes, maturity date	2014
3.45% Senior Notes due August 2015 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	3.45%
Senior notes, maturity date	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
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
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 2042 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	5.25%
Senior notes, maturity date	2042