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Fair Value of Financial Instruments (Carrying Values and Fair Values of Debt Instruments) (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Carrying value of senior notes $ 2,075,260 $ 2,157,285
Senior Notes due 2016 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes 100,000 100,000
Fair value of senior notes [1] $ 100,006 $ 100,536
Interest rate of senior notes 3.30% 3.30%
Senior note year due 2016  
Senior Notes due 2018 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 50,000 $ 50,000
Fair value of senior notes [1] $ 52,005 $ 51,860
Interest rate of senior notes 4.00% 4.00%
Senior note year due 2018  
Senior Notes due 2019 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 175,000 $ 175,000
Fair value of senior notes [1] $ 193,070 $ 190,985
Interest rate of senior notes 5.25% 5.25%
Senior note year due 2019  
Senior Notes due 2021 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 100,000 $ 100,000
Fair value of senior notes [1] $ 109,872 $ 107,613
Interest rate of senior notes 4.64% 4.64%
Senior note year due 2021  
Senior Notes due 2022 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 125,000 $ 125,000
Fair value of senior notes [1] $ 127,477 $ 123,516
Interest rate of senior notes 3.09% 3.09%
Senior note year due 2022  
Senior Notes due 2025 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes $ 375,000 $ 375,000
Fair value of senior notes [1] $ 384,754 $ 370,245
Interest rate of senior notes 3.41% 3.41%
Senior note year due 2025  
[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.