Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 138,874
Due after one year through five years	729,234
Due after five years through ten years	1,008,886
Due after ten years	2,757,291
Amortized Cost	5,188,994	$ 4,941,944
Fair Value
Due in one year or less	140,388
Due after one year through five years	725,360
Due after five years through ten years	1,004,600
Due after ten years	2,791,482
Fair Value	5,214,544	$ 5,223,302
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	171,018
Fair Value
Debt Maturities, without Single Maturity Date	172,827
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	282,300
Fair Value
Debt Maturities, without Single Maturity Date	278,108
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	101,391
Fair Value
Debt Maturities, without Single Maturity Date	$ 101,779

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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