Debt (Summary of Senior Notes) (Details) - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Senior note carrying value $ 2,103,821 $ 1,576,082
Net premiums, discounts, and issuance costs [1] (19,179) (12,163)
Estimated fair value $ 2,204,630 $ 1,643,651
Unsecured Senior Notes 6.50% due May 2016    
Debt Instrument [Line Items]    
Stated interest rate 6.50% 6.50%
Senior note carrying value [2] $ 0 $ 465,245
Unsecured Senior Notes 7.625% due October 2017    
Debt Instrument [Line Items]    
Stated interest rate 7.625% 7.625%
Senior note carrying value [3] $ 123,000 $ 123,000
4.250% unsecured senior notes due March 2021    
Debt Instrument [Line Items]    
Stated interest rate 4.25%  
Senior note carrying value [2] $ 300,000 0
5.500% unsecured senior notes due March 2026    
Debt Instrument [Line Items]    
Stated interest rate 5.50%  
Senior note carrying value [2] $ 700,000 $ 0
Unsecured Senior Notes 7.875% due June 2032    
Debt Instrument [Line Items]    
Stated interest rate 7.875% 7.875%
Senior note carrying value [2] $ 300,000 $ 300,000
Unsecured Senior Notes 6.375% due May 2033    
Debt Instrument [Line Items]    
Stated interest rate 6.375% 6.375%
Senior note carrying value [2] $ 400,000 $ 400,000
Unsecured Senior Notes 6.00% due February 2035    
Debt Instrument [Line Items]    
Stated interest rate 6.00% 6.00%
Senior note carrying value [2] $ 300,000 $ 300,000
Senior Notes | Accounting Standards Update 2015-03    
Debt Instrument [Line Items]    
Net premiums, discounts, and issuance costs   10,300
Other Assets | Accounting Standards Update 2015-03    
Debt Instrument [Line Items]    
Net premiums, discounts, and issuance costs   $ (10,300)
[1] The carrying value of senior notes reflects the impact of premiums, discounts, and issuance costs that are amortized to interest cost over the respective terms of the senior notes. As discussed in Note 1, we adopted ASU 2015-03 in January 2016. We applied the new guidance retrospectively to all prior periods presented in the financial statements to conform to the 2016 presentation. As a result, $10.3 million of debt issuance costs at December 31, 2015, were reclassified from other assets to a reduction in senior notes.
[2] Redeemable prior to maturity; guaranteed on a senior basis by certain wholly-owned subsidiaries.
[3] Not redeemable prior to maturity; guaranteed on a senior basis by certain wholly-owned subsidiaries.