Debt - Schedule of Debt (Supplemental) (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
2.50% Senior Notes due March 2017 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes			2.50%
5.75% Senior Notes due March 2018 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	5.75%
Senior notes, maturity date	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%			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
7.75% Senior Notes due January 2024 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	7.75%
Senior notes, maturity date	2024
7.70% Senior Notes due April 2025 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	7.70%
Senior notes, maturity date	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
8.70% Senior Notes due April 2045 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	8.70%
Senior notes, maturity date	2045
Senior Notes [Member] | 2.50% Senior Notes due March 2017 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes		2.50%
Senior notes, maturity date		2017
Senior Notes [Member] | 5.75% Senior Notes due March 2018 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	5.75%
Senior notes, maturity date	2018
Senior Notes [Member] | 7.50% Senior Notes due March 2019 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	7.50%
Senior notes, maturity date	2019
Senior Notes [Member] | 4.90% Senior Notes due August 2020 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	4.90%	4.90%		4.90%
Senior notes, maturity date	2020
Senior Notes [Member] | 4.625% Senior Notes due March 2021 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	4.625%	4.625%
Senior notes, maturity date	2021
Senior Notes [Member] | 3.95% Senior Notes due March 2022 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	3.95%	3.95%
Senior notes, maturity date	2022
Senior Notes [Member] | 7.75% Senior Notes due January 2024 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	7.75%
Senior notes, maturity date	2024
Senior Notes [Member] | 7.70% Senior Notes due April 2025 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	7.70%
Senior notes, maturity date	2025
Senior Notes [Member] | 6.20% Senior Notes due August 2040 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	6.20%
Senior notes, maturity date	2040
Senior Notes [Member] | 6.05% Senior Notes due March 2041 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	6.05%
Senior notes, maturity date	2041
Senior Notes [Member] | 5.25% Senior Notes due March 2042 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	5.25%
Senior notes, maturity date	2042
Senior Notes [Member] | 8.70% Senior Notes due April 2045 [Member]
Debt Instrument [Line Items]
Interest rate on senior notes	8.70%
Senior notes, maturity date	2045