Debt (Summary of Senior Notes) (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Senior note carrying value	[1]	$ 1,583,580	$ 1,818,561
Estimated fair value		$ 1,659,964	$ 1,952,774
Unsecured Senior Notes 5.25% due June 2015
Debt Instrument [Line Items]
Stated interest rate		5.25%	5.25%
Senior note carrying value	[2]	$ 0	$ 236,452
Debt Instrument, Maturity Date, Description		June 2015	June 2015
Unsecured Senior Notes 6.50% due May 2016
Debt Instrument [Line Items]
Stated interest rate		6.50%	6.50%
Senior note carrying value	[2]	$ 463,222	$ 462,009
Debt Instrument, Maturity Date, Description		May 2016	May 2016
Unsecured Senior Notes 7.625% due October 2017
Debt Instrument [Line Items]
Stated interest rate		7.625%	7.625%
Senior note carrying value	[3]	$ 122,797	$ 122,752
Debt Instrument, Maturity Date, Description		October 2017	October 2017
Unsecured Senior Notes 7.875% due June 2032
Debt Instrument [Line Items]
Stated interest rate		7.875%	7.875%
Senior note carrying value	[2]	$ 299,261	$ 299,239
Debt Instrument, Maturity Date, Description		June 2032	June 2032
Unsecured Senior Notes 6.375% due May 2033
Debt Instrument [Line Items]
Stated interest rate		6.375%	6.375%
Senior note carrying value	[2]	$ 398,677	$ 398,640
Debt Instrument, Maturity Date, Description		May 2033	May 2033
Unsecured Senior Notes 6.00% due February 2035
Debt Instrument [Line Items]
Stated interest rate		6.00%	6.00%
Senior note carrying value	[2]	$ 299,623	$ 299,469
Debt Instrument, Maturity Date, Description		February 2035	February 2035

[1] The recorded carrying value reflects the impact of various discounts and premiums that are amortized to interest cost over the respective terms of the 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.