Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 162,744
Due after one year through five years	701,515
Due after five years through ten years	1,005,349
Due after ten years	3,164,130
Amortized Cost	5,623,693	$ 5,244,903
Fair Value
Due in one year or less	164,151
Due after one year through five years	707,518
Due after five years through ten years	1,046,082
Due after ten years	3,430,304
Fair Value	5,956,391	$ 5,199,595
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	139,728
Fair Value
Debt Maturities, without Single Maturity Date	140,922
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	366,856
Fair Value
Debt Maturities, without Single Maturity Date	381,582
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	83,371
Fair Value
Debt Maturities, without Single Maturity Date	$ 85,832

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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