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Fair Value Measurements - Fair Values of Financial Instruments not Recorded at Fair Value (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
5% Senior Notes due 2022 [Member]    
Fair Value Measurements [Line Items]    
Debt instrument, maturity date 2022  
Debt instrument, stated interest rate 5.00%  
4 1/2% Senior Notes due 2023 [Member]    
Fair Value Measurements [Line Items]    
Debt instrument, maturity date 2023  
Debt instrument, stated interest rate 4.50%  
3.8% Senior Notes due 2024 [Member]    
Fair Value Measurements [Line Items]    
Debt instrument, maturity date 2024  
Debt instrument, stated interest rate 3.80%  
Senior notes   $ 992,036
4.375% Senior Notes Due 2028    
Fair Value Measurements [Line Items]    
Debt instrument, maturity date 2028  
Debt instrument, stated interest rate 4.375%  
Senior notes   988,061
4.9% Senior Notes due 2044 [Member]    
Fair Value Measurements [Line Items]    
Debt instrument, maturity date 2044  
Debt instrument, stated interest rate 4.90%  
Senior notes   691,354
Carrying Amount [Member]    
Fair Value Measurements [Line Items]    
Revolving credit facility $ 0 188,000
Note payable 7,700 9,974
Total debt 5,768,349 6,353,691
Carrying Amount [Member] | 5% Senior Notes due 2022 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 1,598,404 1,997,576
Carrying Amount [Member] | 4 1/2% Senior Notes due 2023 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 1,488,960 1,486,690
Carrying Amount [Member] | 3.8% Senior Notes due 2024 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 993,151 992,036
Carrying Amount [Member] | 4.375% Senior Notes Due 2028    
Fair Value Measurements [Line Items]    
Senior notes 988,617 988,061
Carrying Amount [Member] | 4.9% Senior Notes due 2044 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 691,517 691,354
Fair Value [Member]    
Fair Value Measurements [Line Items]    
Revolving credit facility 0 188,000
Note payable 7,700 9,900
Total debt 5,583,700 6,420,600
Fair Value [Member] | 5% Senior Notes due 2022 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 1,590,900 2,040,000
Fair Value [Member] | 4 1/2% Senior Notes due 2023 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 1,476,300 1,526,800
Fair Value [Member] | 3.8% Senior Notes due 2024 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 947,200 988,800
Fair Value [Member] | 4.375% Senior Notes Due 2028    
Fair Value Measurements [Line Items]    
Senior notes 942,800 987,200
Fair Value [Member] | 4.9% Senior Notes due 2044 [Member]    
Fair Value Measurements [Line Items]    
Senior notes $ 618,800 $ 679,900