Investments - Schedule of Fixed Maturities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 143,830
Due after one year through five years	578,037
Due after five years through ten years	1,434,971
Due after ten years through twenty years	4,363,391
Due after twenty years	7,944,279
Mortgage-backed and asset-backed securities	187,043
Cost or Amortized Cost	14,651,551	$ 14,188,050
Fair Value
Due in one year or less	148,150
Due after one year through five years	622,103
Due after five years through ten years	1,597,721
Due after ten years through twenty years	5,019,819
Due after twenty years	8,733,397
Mortgage-backed and asset-backed securities	197,096
Fixed Maturities, Available for Sale, Fair Value	$ 16,318,286	$ 15,245,861