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Debt (Summary of Senior Notes) (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Debt Instrument [Line Items]    
Senior note carrying value $ 2,511,729 [1] $ 2,509,613 [1]
Unsecured Senior Notes 5.25% due January 2014 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 187,973 [2] 187,970 [2]
Interest Rate, Stated Percentage 5.25% 5.25%
Maturity Date, Description January 2014 January 2014
Unsecured Senior Notes 5.70% due May 2014 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 208,745 [2] 208,274 [2]
Interest Rate, Stated Percentage 5.70% 5.70%
Maturity Date, Description May 2014 May 2014
Unsecured Senior Notes 5.20% due February 2015 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 95,620 [2] 95,615 [2]
Interest Rate, Stated Percentage 5.20% 5.20%
Maturity Date, Description February 2015 February 2015
Unsecured Senior Notes 5.25% due June 2015 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 265,025 [2] 264,058 [2]
Interest Rate, Stated Percentage 5.25% 5.25%
Maturity Date, Description June 2015 June 2015
Unsecured Senior Notes 6.50% due May 2016 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 457,761 [2] 457,154 [2]
Interest Rate, Stated Percentage 6.50% 6.50%
Maturity Date, Description May 2016 May 2016
Unsecured Senior Notes 7.625% due October 2017 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 149,508 [3] 149,481 [3]
Interest Rate, Stated Percentage 7.625% 7.625%
Maturity Date, Description October 2017 October 2017
Unsecured Senior Notes 7.875% due June 2032 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 299,163 [2] 299,152 [2]
Interest Rate, Stated Percentage 7.875% 7.875%
Maturity Date, Description June 2032 June 2032
Unsecured Senior Notes 6.375% due May 2033 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 398,511 [2] 398,492 [2]
Interest Rate, Stated Percentage 6.375% 6.375%
Maturity Date, Description May 2033 May 2033
Unsecured Senior Notes 6.00% due February 2035 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 299,423 [2] 299,417 [2]
Interest Rate, Stated Percentage 6.00% 6.00%
Maturity Date, Description February 2035 February 2035
Unsecured Senior Notes 7.375% due June 2046 [Member]
    
Debt Instrument [Line Items]    
Senior note carrying value 150,000 [2] 150,000 [2]
Interest Rate, Stated Percentage 7.375% 7.375%
Maturity Date, Description June 2046 June 2046
Fair Value, Inputs, Level 2 [Member]
    
Debt Instrument [Line Items]    
Estimated fair value $ 2,675,497 $ 2,663,451
[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.