Investments - Schedule of Fixed Maturities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 106,827
Due after one year through five years	736,487
Due after five years through ten years	1,490,930
Due after ten years through twenty years	5,000,345
Due after twenty years	7,733,027
Mortgage-backed and asset-backed securities	206,874
Cost or Amortized Cost	15,274,490	$ 14,995,101
Fair Value
Due in one year or less	107,973
Due after one year through five years	766,321
Due after five years through ten years	1,640,983
Due after ten years through twenty years	5,690,933
Due after twenty years	8,202,784
Mortgage-backed and asset-backed securities	226,891
Fair Value	$ 16,635,885	$ 16,969,325