Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
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 5.0%
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%	4.50%
Senior Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Interest rate stated percentage	4.65%	4.65%
Senior Notes, due 2025 at 4.5%
Fair Value of Assets and Liabilities
Interest rate stated percentage	4.50%	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
Unamortized discounts	$ (11,935)	$ (12,865)
Financial liabilities, Fair Value	2,421,543	2,420,613
Carrying Amount | Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	275,000	275,000
Carrying Amount | Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	300,000	300,000
Carrying Amount | Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	350,000	350,000
Carrying Amount | Senior Notes, due 2022 at 5.0%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	500,000	500,000
Carrying Amount | Senior Notes, due 2023 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	300,000	300,000
Carrying Amount | Senior Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	350,000	350,000
Carrying Amount | Senior Notes, due 2025 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	350,000	350,000
Carrying Amount | Convertible Senior Notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	8,478	8,478
Fair Value
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	2,502,721	2,555,093
Fair Value | Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	281,035	287,680
Fair Value | Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	317,970	321,675
Fair Value | Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	379,888	386,755
Fair Value | Senior Notes, due 2022 at 5.0%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	521,988	529,160
Fair Value | Senior Notes, due 2023 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	299,435	306,516
Fair Value | Senior Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	349,272	360,757
Fair Value | Senior Notes, due 2025 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	344,334	353,701
Fair Value | Convertible Senior Notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Financial liabilities, Carrying Amount	$ 8,799	$ 8,849