Fair Value of Assets and Liabilities - Debt Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Senior unsecured notes, due 2018 at 6.70%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	6.70%
Senior Unsecured Notes, due 2021 at 4.25%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.25%
Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	5.00%
Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.50%
Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.65%
Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.50%
Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	5.25%
Senior Unsecured Notes, due 2027 at 4.95%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.95%
Convertible Unsecured Senior Notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Interest rate, stated percentage		3.80%
Carrying amount
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	$ 3,163,865	$ 2,574,386
Carrying amount | Senior unsecured notes, due 2018 at 6.70%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	349,812	349,387
Carrying amount | Senior Unsecured Notes, due 2021 at 4.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	395,137	394,056
Carrying amount | Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	494,095	493,187
Carrying amount | Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	499,063	298,134
Carrying amount | Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	347,383	347,079
Carrying amount | Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	344,883	344,368
Carrying amount | Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	340,544	339,697
Carrying amount | Senior Unsecured Notes, due 2027 at 4.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	392,948	0
Carrying amount | Convertible Unsecured Senior Notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	0	8,478
Fair value
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	3,361,052	2,652,653
Fair value | Senior unsecured notes, due 2018 at 6.70%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	351,253	358,740
Fair value | Senior Unsecured Notes, due 2021 at 4.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	417,246	413,790
Fair value | Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	537,360	527,945
Fair value | Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	530,825	298,845
Fair value | Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	367,026	348,523
Fair value | Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	360,227	341,439
Fair value | Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	375,799	354,772
Fair value | Senior Unsecured Notes, due 2027 at 4.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	421,316	0
Fair value | Convertible Unsecured Senior Notes, due 2027 at 3.8%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	$ 0	$ 8,599