Debt - Estimated Fair Value of Our Long-Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Total long-term debt, Estimated Fair Value	$ 4,811,696	$ 5,672,292
Total long-term debt, Carrying Value	4,737,081	5,556,251
Commercial Paper Program [Member]
Debt Instrument [Line Items]
Total long-term debt, Estimated Fair Value	991,653	1,561,141
Total long-term debt, Carrying Value	991,653	1,561,141
7.375% Senior Notes due March 2014 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value		249,964
Total long-term debt, Estimated Fair Value		253,634
3.45% Senior Notes due August 2015 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	350,000	350,000
Total long-term debt, Estimated Fair Value	357,087	363,019
3.05% Senior Notes due March 2016 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	299,978	299,967
Total long-term debt, Estimated Fair Value	306,752	309,878
2.50% Senior Notes due March 2017 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	299,912	299,886
Total long-term debt, Estimated Fair Value	303,855	302,891
7.50% Senior Notes due March 2019 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	201,695	201,695
Total long-term debt, Estimated Fair Value	232,264	232,839
4.90% Senior Notes due August 2020 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	499,118	499,022
Total long-term debt, Estimated Fair Value	530,788	528,597
4.625% Senior Notes due March 2021 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	399,614	399,576
Total long-term debt, Estimated Fair Value	413,996	413,868
3.95% Senior Notes due March 2022 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	399,242	399,178
Total long-term debt, Estimated Fair Value	391,616	390,520
6.20% Senior Notes due August 2040 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	399,894	399,893
Total long-term debt, Estimated Fair Value	416,991	421,720
6.05% Senior Notes due March 2041 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	397,672	397,646
Total long-term debt, Estimated Fair Value	408,334	417,312
5.25% Senior Notes due March 2042 [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	498,303	498,283
Total long-term debt, Estimated Fair Value	458,360	476,873
Total Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Total senior unsecured notes, Carrying Value	3,745,428	3,995,110
Total long-term debt, Estimated Fair Value	$ 3,820,043	$ 4,111,151