Investments - Schedule of Fixed Maturities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 88,952
Due from one to five years	628,487
Due from five to ten years	1,335,053
Due from ten to twenty years	4,256,853
Due after twenty years	8,127,226
Mortgage-backed and asset-backed securities	165,679
Cost or Amortized Cost	14,602,250	$ 14,188,050
Fair Value
Due in one year or less	91,746
Due from one to five years	675,601
Due from five to ten years	1,474,252
Due from ten to twenty years	4,794,890
Due after twenty years	8,663,281
Mortgage-backed and asset-backed securities	174,361
Fixed Maturities, Available for Sale, Fair Value	$ 15,874,131	$ 15,245,861