Fair Value of Assets and Liabilities (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Senior Notes, due 2014 at 7.875%
Fair Value of Assets and Liabilities
Interest rate stated percentage	7.875%	7.875%
Senior Notes, due 2015 at 5.125%
Fair Value of Assets and Liabilities
Interest rate stated percentage	5.125%	5.125%
Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Interest rate stated percentage	6.30%	6.30%
Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Interest rate stated percentage	5.625%	5.625%
Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Interest rate stated percentage	6.70%	6.70%
Senior Notes, due 2022 at 5%
Fair Value of Assets and Liabilities
Interest rate stated percentage	5.00%	5.00%
Carrying Amount
Fair Value of Assets and Liabilities
Financial liabilities	2,002,850	2,002,358
Unamortized discounts	(10,628)	(11,120)
Carrying Amount | Senior Notes, due 2014 at 7.875%
Fair Value of Assets and Liabilities
Financial liabilities	300,000	300,000
Carrying Amount | Senior Notes, due 2015 at 5.125%
Fair Value of Assets and Liabilities
Financial liabilities	280,000	280,000
Carrying Amount | Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Financial liabilities	275,000	275,000
Carrying Amount | Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Financial liabilities	300,000	300,000
Carrying Amount | Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Financial liabilities	350,000	350,000
Carrying Amount | Senior Notes, due 2022 at 5%
Fair Value of Assets and Liabilities
Financial liabilities	500,000	500,000
Carrying Amount | Convertible senior notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Financial liabilities	8,478	8,478
Fair Value
Fair Value of Assets and Liabilities
Financial liabilities	2,212,641	2,231,265
Fair Value | Senior Notes, due 2014 at 7.875%
Fair Value of Assets and Liabilities
Financial liabilities	324,739	333,360
Fair Value | Senior Notes, due 2015 at 5.125%
Fair Value of Assets and Liabilities
Financial liabilities	295,026	298,926
Fair Value | Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Financial liabilities	310,385	306,078
Fair Value | Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Financial liabilities	331,805	335,173
Fair Value | Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Financial liabilities	412,098	417,293
Fair Value | Senior Notes, due 2022 at 5%
Fair Value of Assets and Liabilities
Financial liabilities	529,555	531,343
Fair Value | Convertible senior notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Financial liabilities	9,033	9,092