Investments - Schedule of Fixed Maturities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 189,867
Due after one year through five years	611,511
Due after five years through ten years	1,686,348
Due after ten years through twenty years	5,023,824
Due after twenty years	7,742,061
Mortgage-backed and asset-backed securities	208,753
Cost or Amortized Cost	15,462,364	$ 14,995,101
Fair Value
Due in one year or less	193,934
Due after one year through five years	632,417
Due after five years through ten years	1,820,952
Due after ten years through twenty years	5,505,111
Due after twenty years	7,848,163
Mortgage-backed and asset-backed securities	230,894
Fair Value	$ 16,231,471	$ 16,969,325