Debt - Estimated Fair Value of Our Total Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Total debt	$ 4,488,901	$ 4,869,020
Senior unsecured notes, Estimated Fair Value	3,260,231	4,540,627
3.45% Senior Notes due August 2015 [Member]
Debt Instrument [Line Items]
Senior unsecured notes		350,000
Senior unsecured notes, Estimated Fair Value		354,992
3.05% Senior Notes due March 2016 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	299,997	299,982
Senior unsecured notes, Estimated Fair Value	299,340	302,515
2.50% Senior Notes due March 2017 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	299,956	299,920
Senior unsecured notes, Estimated Fair Value	284,334	287,014
4.00% Senior Notes due March 2018 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	249,602
Senior unsecured notes, Estimated Fair Value	227,285
7.50% Senior Notes due March 2019 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	201,695	201,695
Senior unsecured notes, Estimated Fair Value	194,273	212,068
4.90% Senior Notes due August 2020 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	499,287	499,151
Senior unsecured notes, Estimated Fair Value	378,761	471,095
4.625% Senior Notes due March 2021 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	399,680	399,627
Senior unsecured notes, Estimated Fair Value	289,450	363,837
3.95% Senior Notes due March 2022 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	399,354	399,264
Senior unsecured notes, Estimated Fair Value	265,643	346,425
5.95% Senior Notes due April 2025 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	448,814
Senior unsecured notes, Estimated Fair Value	308,870
6.20% Senior Notes due August 2040 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	399,896	399,895
Senior unsecured notes, Estimated Fair Value	237,005	350,351
6.05% Senior Notes due March 2041 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	397,719	397,681
Senior unsecured notes, Estimated Fair Value	239,464	343,653
5.25% Senior Notes due March 2042 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	498,338	498,310
Senior unsecured notes, Estimated Fair Value	279,919	385,181
6.95% Senior Notes due April 2045 [Member]
Debt Instrument [Line Items]
Senior unsecured notes	394,563
Senior unsecured notes, Estimated Fair Value	255,887
Total Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Senior unsecured notes	4,488,901	3,745,525
Senior unsecured notes, Estimated Fair Value	$ 3,260,231	3,417,131
Credit Facilities and Commercial Paper Program [Member]
Debt Instrument [Line Items]
Total debt		1,123,495
Senior unsecured notes, Estimated Fair Value		$ 1,123,496