Condensed Consolidating Financial Information - Guarantor Obligations (Detail)	Dec. 31, 2017 USD ($)
5.75% Senior Notes due March 2018
Guarantor Obligations [Line Items]
Face value of senior notes	$ 250,000,000
Interest rate on senior notes	5.75%
7.50% Senior Notes due March 2019
Guarantor Obligations [Line Items]
Face value of senior notes	$ 202,000,000
Interest rate on senior notes	7.50%
4.90% Senior Notes due August 2020
Guarantor Obligations [Line Items]
Face value of senior notes	$ 168,000,000
Interest rate on senior notes	4.90%
4.625% Senior Notes due March 2021
Guarantor Obligations [Line Items]
Face value of senior notes	$ 209,000,000
Interest rate on senior notes	4.625%
3.95% Senior Notes due March 2022
Guarantor Obligations [Line Items]
Face value of senior notes	$ 126,000,000
Interest rate on senior notes	3.95%
7.75% Senior Notes due January 2024
Guarantor Obligations [Line Items]
Face value of senior notes	$ 1,000,000,000
Interest rate on senior notes	7.75%
7.70% Senior Notes due April 2025
Guarantor Obligations [Line Items]
Face value of senior notes	$ 450,000,000
Interest rate on senior notes	7.70%
6.20% Senior Notes due August 2040
Guarantor Obligations [Line Items]
Face value of senior notes	$ 400,000,000
Interest rate on senior notes	6.20%
6.05% Senior Notes due March 2041
Guarantor Obligations [Line Items]
Face value of senior notes	$ 400,000,000
Interest rate on senior notes	6.05%
5.25% Senior Notes due March 2042
Guarantor Obligations [Line Items]
Face value of senior notes	$ 500,000,000
Interest rate on senior notes	5.25%
8.70% Senior Notes due April 2045
Guarantor Obligations [Line Items]
Face value of senior notes	$ 400,000,000
Interest rate on senior notes	8.70%