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Debt - Schedule of Debt (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Total debt $ 4,488,901 $ 4,869,020
Less: Current maturities of long-term debt (299,997)  
Long-term debt 4,188,904 4,869,020
Credit Facilities and Commercial Paper Program [Member]    
Debt Instrument [Line Items]    
Total debt   1,123,495
3.45% Senior Notes due August 2015 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes   350,000
3.05% Senior Notes due March 2016 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 299,997 299,982
2.50% Senior Notes due March 2017 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 299,956 299,920
4.00% Senior Notes due March 2018 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 249,602  
7.50% Senior Notes due March 2019 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 201,695 201,695
4.90% Senior Notes due August 2020 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 499,287 499,151
4.625% Senior Notes due March 2021 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 399,680 399,627
3.95% Senior Notes due March 2022 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 399,354 399,264
5.95% Senior Notes due April 2025 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 448,814  
6.20% Senior Notes due August 2040 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 399,896 399,895
6.05% Senior Notes due March 2041 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 397,719 397,681
5.25% Senior Notes due March 2042 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 498,338 498,310
6.95% Senior Notes due April 2045 [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes 394,563  
Total Senior Unsecured Notes [Member]    
Debt Instrument [Line Items]    
Senior unsecured notes $ 4,488,901 $ 3,745,525