Investments - Schedule of Fixed Maturities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 80,564
Due after one year through five years	732,395
Due after five years through ten years	1,611,476
Due after ten years through twenty years	5,051,297
Due after twenty years	7,664,183
Mortgage-backed and asset-backed securities	210,237
Cost or Amortized Cost	15,350,152	$ 14,995,101
Fair Value
Due in one year or less	81,901
Due after one year through five years	757,040
Due after five years through ten years	1,745,491
Due after ten years through twenty years	5,607,509
Due after twenty years	7,862,890
Mortgage-backed and asset-backed securities	229,860
Fair Value	$ 16,284,691	$ 16,969,325