Fair Value of Assets and Liabilities (Details 2) - USD ($) $ in Thousands	Dec. 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 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	$ 2,574,386		$ 2,411,917
Carrying Amount | Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	0		274,869
Carrying Amount | Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	0		299,576
Carrying Amount | Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	349,387		348,821
Carrying Amount | Senior Notes, due 2021 at 4.25%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	394,056		0
Carrying Amount | Senior Notes, due 2022 at 5.0%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	493,187		491,974
Carrying Amount | Senior Notes, due 2023 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	298,134		297,845
Carrying Amount | Senior Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	347,079		346,674
Carrying Amount | Senior Notes, due 2025 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	344,368		343,680
Carrying Amount | Senior Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	339,697		0
Carrying Amount | Convertible senior notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	8,478		8,478
Fair Value
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	2,652,653		2,462,034
Fair Value | Senior Notes, due 2016 at 6.3%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	0		275,813
Fair Value | Senior Notes, due 2017 at 5.625%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	0		311,181
Fair Value | Senior Notes, due 2018 at 6.7%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	358,740		370,438
Fair Value | Senior Notes, due 2021 at 4.25%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	413,790		0
Fair Value | Senior Notes, due 2022 at 5.0%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	527,945		515,760
Fair Value | Senior Notes, due 2023 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	298,845		295,709
Fair Value | Senior Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	348,523		346,010
Fair Value | Senior Notes, due 2025 at 4.5%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	341,439		338,426
Fair Value | Senior Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	354,772		0
Fair Value | Convertible senior notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Financial liabilities, Fair Value	$ 8,599		$ 8,697