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Fair Value of Financial Instruments (Carrying Values and Fair Values of Debt Instruments) (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Carrying value of senior notes $ 3,763,330 $ 3,926,321
Senior Notes [Member] | Senior Notes due 2018 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes   50,000
Fair value of senior notes [1]   50,223
Senior note year due 2018  
Senior Notes [Member] | Senior Notes due 2019 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 175,000 175,000
Fair value of senior notes [1] $ 178,150 182,547
Senior note year due 2019  
Senior Notes [Member] | Senior Notes due 2021 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 100,000 100,000
Fair value of senior notes [1] $ 101,780 104,985
Senior note year due 2021  
Senior Notes [Member] | New Senior Notes due 2021 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 150,000 150,000
Fair value of senior notes [1] $ 144,299 146,855
Senior note year due 2021  
Senior Notes [Member] | Senior Notes due 2022 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 125,000 125,000
Fair value of senior notes [1] $ 121,062 124,532
Senior note year due 2022  
Senior Notes [Member] | Senior Notes due 2023 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 200,000 200,000
Fair value of senior notes [1] $ 188,988 194,660
Senior note year due 2023  
Senior Notes [Member] | Senior Notes due 2024 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 150,000 150,000
Fair value of senior notes [1] $ 143,651 149,133
Senior note year due 2024  
Senior Notes [Member] | Senior Notes due 2025 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 375,000 375,000
Fair value of senior notes [1] $ 358,141 375,311
Senior note year due 2025  
Senior Notes [Member] | Senior Notes due 2026 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 400,000 400,000
Fair value of senior notes [1] $ 369,921 388,760
Senior note year due 2026  
Senior Notes [Member] | Senior Notes due 2027 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 250,000 250,000
Fair value of senior notes [1] $ 236,409 $ 250,029
Senior note year due 2027  
[1] Senior Notes are classified as Level 2 within the fair value hierarchy. Fair value is based on quotes of bonds with similar ratings in similar industries.