Fair Value of Assets and Liabilities - Debt Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Mar. 11, 2016	Dec. 31, 2015
Senior Unsecured Notes, due 2016 at 6.30% (2)
Fair Value of Assets and Liabilities
Interest rate, stated percentage	6.30%	6.30%	6.30%
Senior Unsecured Notes, due 2017 at 5.625% (3)
Fair Value of Assets and Liabilities
Interest rate, stated percentage	5.625%		5.625%
Senior Unsecured Notes, due 2018 at 6.70%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	6.70%		6.70%
Senior Unsecured Notes, due 2021 at 4.25% (4)
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.25%		4.25%
Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	5.00%		5.00%
Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.50%		4.50%
Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.65%		4.65%
Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.50%		4.50%
Senior Unsecured Notes, due 2026 at 5.25% (4)
Fair Value of Assets and Liabilities
Interest rate, stated percentage	5.25%		5.25%
Convertible Unsecured Senior Notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	3.80%		3.80%
Fair value
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	$ 2,744,336		$ 2,462,034
Fair value | Senior Unsecured Notes, due 2016 at 6.30% (2)
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	0		275,813
Fair value | Senior Unsecured Notes, due 2017 at 5.625% (3)
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	0		311,181
Fair value | Senior Unsecured Notes, due 2018 at 6.70%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	363,426		370,438
Fair value | Senior Unsecured Notes, due 2021 at 4.25% (4)
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	425,874		0
Fair value | Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	544,650		515,760
Fair value | Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	311,609		295,709
Fair value | Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	361,205		346,010
Fair value | Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	356,846		338,426
Fair value | Senior Unsecured Notes, due 2026 at 5.25% (4)
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	372,173		0
Fair value | Convertible Unsecured Senior Notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	8,553		8,697
Carrying amount
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	2,572,956		2,411,917
Carrying amount | Senior Unsecured Notes, due 2016 at 6.30% (2)
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	0		274,869
Carrying amount | Senior Unsecured Notes, due 2017 at 5.625% (3)
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	0		299,576
Carrying amount | Senior Unsecured Notes, due 2018 at 6.70%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	349,246		348,821
Carrying amount | Senior Unsecured Notes, due 2021 at 4.25% (4)
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	393,697		0
Carrying amount | Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	492,884		491,974
Carrying amount | Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	298,062		297,845
Carrying amount | Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	346,978		346,674
Carrying amount | Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	344,196		343,680
Carrying amount | Senior Unsecured Notes, due 2026 at 5.25% (4)
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	339,415		0
Carrying amount | Convertible Unsecured Senior Notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	$ 8,478		$ 8,478