Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less	$ 238,403
Due after one year through five years	730,695
Due after five years through ten years	958,937
Due after ten years	3,001,873
Amortized Cost	5,550,300	$ 5,283,266
Fair Value
Due in one year or less	240,471
Due after one year through five years	744,717
Due after five years through ten years	998,183
Due after ten years	3,465,247
Fair Value	6,106,322	5,681,970
Asset-backed securities
Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	122,786
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	120,890
Commercial mortgage-backed securities
Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	437,874
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	472,524
Residential mortgage-backed securities
Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	59,732
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	64,290
Fixed maturities
Amortized Cost
Amortized Cost	5,550,300	5,283,266
Fair Value
Fair Value	6,106,322	5,681,970
Fixed maturities \| Asset-backed securities
Amortized Cost
Amortized Cost	122,786	119,602
Fair Value
Fair Value	120,890	119,936
Fixed maturities \| Commercial mortgage-backed securities
Amortized Cost
Amortized Cost	437,874	367,848
Fair Value
Fair Value	472,524	382,916
Fixed maturities \| Residential mortgage-backed securities
Amortized Cost
Amortized Cost	59,732	60,778
Fair Value
Fair Value	$ 64,290	$ 63,804

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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