Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Amortized Cost
Due in one year or less	$ 518,986
Due after one year through five years	1,364,658
Due after five years through ten years	1,306,356
Due after ten years	4,508,735
Amortized Cost	8,713,464	$ 2,433,626
Fair Value
Due in one year or less	523,796
Due after one year through five years	1,410,184
Due after five years through ten years	1,370,010
Due after ten years	4,802,648
Fair Value	9,157,320	$ 2,524,272
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	470,728
Fair Value
Debt Maturities, without Single Maturity Date	495,523
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	221,299
Fair Value
Debt Maturities, without Single Maturity Date	230,391
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	322,702
Fair Value
Debt Maturities, without Single Maturity Date	$ 324,768