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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, 2017
Dec. 31, 2016
Fair Value Measurements [Line Items]    
Term Loan   $ 500,000
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   990,964
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   0
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   691,199
Carrying Amount [Member]    
Fair Value Measurements [Line Items]    
Revolving credit facility $ 188,000 905,000
Term Loan 0 498,865
Note payable 9,974 12,176
Total debt 6,353,691 6,579,916
Carrying Amount [Member] | 5% Senior Notes due 2022 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 1,997,576 1,997,188
Carrying Amount [Member] | 4 1/2% Senior Notes due 2023 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 1,486,690 1,484,524
Carrying Amount [Member] | 3.8% Senior Notes due 2024 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 992,036 990,964
Carrying Amount [Member] | 4.375% Senior Notes Due 2028    
Fair Value Measurements [Line Items]    
Senior notes 988,061 0
Carrying Amount [Member] | 4.9% Senior Notes due 2044 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 691,354 691,199
Fair Value [Member]    
Fair Value Measurements [Line Items]    
Revolving credit facility 188,000 905,000
Term loan 0 500,000
Note payable 9,900 10,200
Total debt 6,420,600 6,447,400
Fair Value [Member] | 5% Senior Notes due 2022 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 2,040,000 2,020,400
Fair Value [Member] | 4 1/2% Senior Notes due 2023 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 1,526,800 1,474,800
Fair Value [Member] | 3.8% Senior Notes due 2024 [Member]    
Fair Value Measurements [Line Items]    
Senior notes 988,800 929,400
Fair Value [Member] | 4.375% Senior Notes Due 2028    
Fair Value Measurements [Line Items]    
Senior notes 987,200 0
Fair Value [Member] | 4.9% Senior Notes due 2044 [Member]    
Fair Value Measurements [Line Items]    
Senior notes $ 679,900 $ 607,600