Fair Value of Assets and Liabilities - Debt Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Senior Unsecured Notes, due 2021 at 4.25%
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%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	5.25%	5.25%
Senior Unsecured Notes, due 2027 at 4.95%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.95%	4.95%
Senior Unsecured Notes, due 2028 at 3.95%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	3.95%	3.95%
Senior Unsecured Notes, due 2030 at 4.375%
Fair Value of Assets and Liabilities
Interest rate, stated percentage	4.375%
Carrying amount
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	$ 3,592,291	$ 3,203,962
Carrying amount | Senior Unsecured Notes, due 2021 at 4.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	395,858	395,497
Carrying amount | Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	494,701	494,398
Carrying amount | Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	499,145	499,104
Carrying amount | Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	347,586	347,484
Carrying amount | Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	345,227	345,055
Carrying amount | Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	341,108	340,826
Carrying amount | Senior Unsecured Notes, due 2027 at 4.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	393,325	393,137
Carrying amount | Senior Unsecured Notes, due 2028 at 3.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	388,748	388,461
Carrying amount | Senior Unsecured Notes, due 2030 at 4.375%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	386,593	0
Fair value
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	3,686,930	3,394,527
Fair value | Senior Unsecured Notes, due 2021 at 4.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	407,652	413,676
Fair value | Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	520,658	533,908
Fair value | Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	512,190	523,275
Fair value | Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	357,635	368,804
Fair value | Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	353,658	363,589
Fair value | Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	364,525	377,431
Fair value | Senior Unsecured Notes, due 2027 at 4.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	408,398	422,914
Fair value | Senior Unsecured Notes, due 2028 at 3.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	376,284	390,930
Fair value | Senior Unsecured Notes, due 2030 at 4.375%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	$ 385,930	$ 0