Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Amortized Cost
Due in one year or less	$ 447,470
Due after one year through five years	637,263
Due after five years through ten years	491,903
Due after ten years	387,539
Amortized Cost	2,521,513	$ 2,433,626
Fair Value
Due in one year or less	449,492
Due after one year through five years	662,980
Due after five years through ten years	530,852
Due after ten years	432,330
Fair Value	2,653,708	$ 2,524,272
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	270,865
Fair Value
Debt Maturities, without Single Maturity Date	285,103
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	124,307
Fair Value
Debt Maturities, without Single Maturity Date	130,423
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	162,166
Fair Value
Debt Maturities, without Single Maturity Date	$ 162,528