Investments - Schedule of Fixed Maturities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Amortized Cost
Due in one year or less	$ 14,764
Due from one to five years	625,222
Due from five to ten years	1,149,329
Due from ten to twenty years	4,071,596
Due after twenty years	7,964,507
Mortgage-backed and asset-backed securities	119,049
Fixed Maturities, Available for Sale, Amortized Cost	13,944,467	$ 13,251,871
Fair Value
Due in one year or less	14,920
Due from one to five years	680,902
Due from five to ten years	1,299,091
Due from ten to twenty years	4,751,987
Due after twenty years	8,966,796
Mortgage-backed and asset-backed securities	124,004
Fixed Maturities, Available for Sale, Fair Value	$ 15,837,700	$ 13,758,024