Investments - Schedule of Fixed Maturities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 140,702
Due after one year through five years	660,487
Due after five years through ten years	1,430,894
Due after ten years through twenty years	4,397,807
Due after twenty years	8,079,191
Mortgage-backed and asset-backed securities	205,499
Cost or Amortized Cost	14,914,580	$ 14,188,050
Fair Value
Due in one year or less	143,863
Due after one year through five years	705,847
Due after five years through ten years	1,604,091
Due after ten years through twenty years	5,084,516
Due after twenty years	8,893,450
Mortgage-backed and asset-backed securities	221,146
Fair Value	$ 16,652,913	$ 15,245,861