Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 188,789
Due after one year through five years	738,939
Due after five years through ten years	1,035,613
Due after ten years	2,581,645
Amortized Cost	5,107,015	$ 4,941,944
Fair Value
Due in one year or less	191,560
Due after one year through five years	747,181
Due after five years through ten years	1,064,058
Due after ten years	2,694,941
Fair Value	5,258,693	$ 5,223,302
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	179,037
Fair Value
Debt Maturities, without Single Maturity Date	181,610
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	316,110
Fair Value
Debt Maturities, without Single Maturity Date	311,352
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	66,882
Fair Value
Debt Maturities, without Single Maturity Date	$ 67,991