Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Amortized Cost
Due in one year or less	$ 476,623
Due after one year through five years	1,403,762
Due after five years through ten years	1,383,206
Due after ten years	5,800,150
Amortized Cost	10,078,018	$ 2,433,626
Fair Value
Due in one year or less	480,670
Due after one year through five years	1,447,830
Due after five years through ten years	1,460,019
Due after ten years	6,056,318
Fair Value	10,494,559	$ 2,524,272
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	319,989
Fair Value
Debt Maturities, without Single Maturity Date	322,676
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	485,170
Fair Value
Debt Maturities, without Single Maturity Date	508,808
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	209,118
Fair Value
Debt Maturities, without Single Maturity Date	$ 218,238