Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 531,084
Due after one year through five years	1,250,571
Due after five years through ten years	1,382,016
Due after ten years	5,928,543
Amortized Cost	10,077,628	$ 9,818,298
Fair Value
Due in one year or less	536,561
Due after one year through five years	1,279,220
Due after five years through ten years	1,444,380
Due after ten years	5,660,349
Fair Value	9,915,916	$ 9,362,763
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	322,294
Fair Value
Debt Maturities, without Single Maturity Date	325,247
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	483,673
Fair Value
Debt Maturities, without Single Maturity Date	487,333
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	179,447
Fair Value
Debt Maturities, without Single Maturity Date	$ 182,826