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Fair Value of Financial Instruments (Carrying Values and Fair Values of Debt Instruments) (Detail) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Carrying value of senior notes $ 1,939,427 $ 1,979,565
Senior Notes due 2015 [Member]    
Debt Instrument [Line Items]    
Carrying value of senior notes 175,000 175,000
Fair value of senior notes [1] $ 177,301 $ 181,476
Interest rate of senior notes 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
Fair value of senior notes [1] $ 101,500 $ 102,253
Interest rate of senior notes 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
Fair value of senior notes [1] $ 52,494 $ 52,500
Interest rate of senior notes 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
Fair value of senior notes [1] $ 194,370 $ 192,974
Interest rate of senior notes 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
Fair value of senior notes [1] $ 108,013 $ 108,088
Interest rate of senior notes 4.64% 4.64%
Senior note year due 2021 2021
[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.