Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Mar. 11, 2016	Dec. 31, 2015
Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Interest rate stated percentage	6.30%	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 2021 at 4.25 %
Fair Value of Assets and Liabilities
Interest rate stated percentage	4.25%		4.25%
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%
Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Interest rate stated percentage	5.25%		5.25%
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, Fair Value	$ 2,869,771		$ 2,411,918
Carrying Amount | Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Financial liabilities			274,869
Carrying Amount | Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Financial liabilities	299,666		299,576
Carrying Amount | Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Financial liabilities	348,962		348,821
Carrying Amount | Senior Notes, due 2021 at 4.25 %
Fair Value of Assets and Liabilities
Financial liabilities	392,985
Carrying Amount | Senior Notes, due 2022 at 5.0%
Fair Value of Assets and Liabilities
Financial liabilities	492,278		491,975
Carrying Amount | Senior Notes, due 2023 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities	297,917		297,845
Carrying Amount | Senior Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Financial liabilities	346,775		346,674
Carrying Amount | Senior Notes, due 2025 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities	343,852		343,680
Carrying Amount | Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Financial liabilities	338,858
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, Fair Value	2,953,436		2,462,034
Fair Value | Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Financial liabilities			275,813
Fair Value | Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Financial liabilities	310,418		311,181
Fair Value | Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Financial liabilities	367,199		370,438
Fair Value | Senior Notes, due 2021 at 4.25 %
Fair Value of Assets and Liabilities
Financial liabilities	404,708
Fair Value | Senior Notes, due 2022 at 5.0%
Fair Value of Assets and Liabilities
Financial liabilities	522,773		515,760
Fair Value | Senior Notes, due 2023 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities	298,449		295,709
Fair Value | Senior Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Financial liabilities	349,122		346,010
Fair Value | Senior Notes, due 2025 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities	339,477		338,426
Fair Value | Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Financial liabilities	352,725
Fair Value | Convertible Senior Notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Financial liabilities	$ 8,565		$ 8,697