Investments - Schedule of Fixed Maturities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 204,444
Due after one year through five years	611,042
Due after five years through ten years	1,643,633
Due after ten years through twenty years	5,210,071
Due after twenty years	8,114,988
Mortgage-backed and asset-backed securities	204,050
Cost or Amortized Cost	15,988,228	$ 15,753,471
Fair Value
Due in one year or less	206,980
Due after one year through five years	643,173
Due after five years through ten years	1,814,976
Due after ten years through twenty years	5,843,176
Due after twenty years	8,488,204
Mortgage-backed and asset-backed securities	227,076
Fair Value	$ 17,223,585	$ 16,297,932