Fair Value of Assets and Liabilities (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 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%
Senior Notes, due 2023 at 4.5%
Fair Value of Assets and Liabilities
Interest rate stated percentage	4.50%
Convertible senior notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Interest rate stated percentage	3.80%	3.80%
Carrying Amount
Fair Value of Assets and Liabilities
Financial liabilities	$ 2,302,850	$ 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 | Senior Notes, due 2023 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities	300,000
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	2,459,572	2,231,265
Fair Value | Senior Notes, due 2014 at 7.875%
Fair Value of Assets and Liabilities
Financial liabilities	326,191	333,360
Fair Value | Senior Notes, due 2015 at 5.125%
Fair Value of Assets and Liabilities
Financial liabilities	295,893	298,926
Fair Value | Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Financial liabilities	300,273	306,078
Fair Value | Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Financial liabilities	328,136	335,173
Fair Value | Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Financial liabilities	404,353	417,293
Fair Value | Senior Notes, due 2022 at 5%
Fair Value of Assets and Liabilities
Financial liabilities	505,412	531,343
Fair Value | Senior Notes, due 2023 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities	290,389
Fair Value | Convertible senior notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Financial liabilities	$ 8,925	$ 9,092