Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 409,701
Due after one year through five years	1,160,347
Due after five years through ten years	1,516,165
Due after ten years	5,992,513
Amortized Cost	10,118,639	$ 9,818,298
Fair Value
Due in one year or less	411,603
Due after one year through five years	1,188,688
Due after five years through ten years	1,592,338
Due after ten years	5,724,622
Fair Value	9,968,944	$ 9,362,763
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	366,108
Fair Value
Debt Maturities, without Single Maturity Date	369,601
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	503,210
Fair Value
Debt Maturities, without Single Maturity Date	507,913
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	170,595
Fair Value
Debt Maturities, without Single Maturity Date	$ 174,179