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Fair Value of Financial Instruments (Carrying Values and Fair Values of Debt Instruments) (Detail) - USD ($)
$ in Thousands
1 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2015
Sep. 30, 2015
Dec. 31, 2014
Debt Instrument [Line Items]      
Carrying value of senior notes $ 1,958,716 $ 1,958,716 $ 1,979,565
Senior Notes due 2015 [Member]      
Debt Instrument [Line Items]      
Carrying value of senior notes 175,000 175,000 175,000
Fair value of senior notes [1] $ 175,000 $ 175,000 $ 181,476
Interest rate of senior notes 6.22% 6.22% 6.22%
Senior note year due   2015 2015
Senior Notes due 2016 [Member]      
Debt Instrument [Line Items]      
Carrying value of senior notes $ 100,000 $ 100,000 $ 100,000
Fair value of senior notes [1] $ 101,067 $ 101,067 $ 102,253
Interest rate of senior notes 3.30% 3.30% 3.30%
Senior note year due   2016 2016
Senior Notes due 2018 [Member]      
Debt Instrument [Line Items]      
Carrying value of senior notes $ 50,000 $ 50,000 $ 50,000
Fair value of senior notes [1] $ 52,329 $ 52,329 $ 52,500
Interest rate of senior notes 4.00% 4.00% 4.00%
Senior note year due   2018 2018
Senior Notes due 2019 [Member]      
Debt Instrument [Line Items]      
Carrying value of senior notes $ 175,000 $ 175,000 $ 175,000
Fair value of senior notes [1] $ 193,523 $ 193,523 $ 192,974
Interest rate of senior notes 5.25% 5.25% 5.25%
Senior note year due   2019 2019
Senior Notes due 2021 [Member]      
Debt Instrument [Line Items]      
Carrying value of senior notes $ 100,000 $ 100,000 $ 100,000
Fair value of senior notes [1] $ 109,030 $ 109,030 $ 108,088
Interest rate of senior notes 4.64% 4.64% 4.64%
Senior note year due   2021 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] $ 124,802 $ 124,802
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] $ 372,829 $ 372,829
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.