Fair Value of Assets and Liabilities - Debt Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
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%	4.375%
Carrying amount
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	$ 3,600,314	$ 3,598,295
Carrying amount | Senior Unsecured Notes, due 2021 at 4.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	397,299	396,938
Carrying amount | Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	495,912	495,609
Carrying amount | Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	499,309	499,268
Carrying amount | Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	347,991	347,890
Carrying amount | Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	345,915	345,743
Carrying amount | Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	342,237	341,955
Carrying amount | Senior Unsecured Notes, due 2027 at 4.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	394,082	393,893
Carrying amount | Senior Unsecured Notes, due 2028 at 3.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	389,897	389,610
Carrying amount | Senior Unsecured Notes, due 2030 at 4.375%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	387,672	387,389
Fair value
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	3,640,136	3,583,452
Fair value | Senior Unsecured Notes, due 2021 at 4.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	402,648	404,582
Fair value | Senior Unsecured Notes, due 2022 at 5.00%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	517,915	510,658
Fair value | Senior Unsecured Notes, due 2023 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	511,155	503,295
Fair value | Senior Unsecured Notes, due 2024 at 4.65%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	357,231	349,741
Fair value | Senior Unsecured Notes, due 2025 at 4.50%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	354,552	341,114
Fair value | Senior Unsecured Notes, due 2026 at 5.25%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	358,311	354,060
Fair value | Senior Unsecured Notes, due 2027 at 4.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	399,208	391,660
Fair value | Senior Unsecured Notes, due 2028 at 3.95%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	368,594	361,232
Fair value | Senior Unsecured Notes, due 2030 at 4.375%
Fair Value of Assets and Liabilities
Total financial liabilities, fair value	$ 370,522	$ 367,110