Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less	$ 204,893
Due after one year through five years	716,192
Due after five years through ten years	971,203
Due after ten years	3,052,484
Amortized Cost	5,624,060	$ 5,283,266
Fair Value
Due in one year or less	207,819
Due after one year through five years	744,822
Due after five years through ten years	1,040,291
Due after ten years	3,566,953
Fair Value	6,280,086	5,681,970
Asset-backed securities
Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	168,513
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	168,220
Commercial mortgage-backed securities
Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	456,095
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	492,958
Residential mortgage-backed securities
Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	54,680
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	59,023
Fixed maturities
Amortized Cost
Amortized Cost	5,624,060	5,283,266
Fair Value
Fair Value	6,280,086	5,681,970
Fixed maturities \| Asset-backed securities
Amortized Cost
Amortized Cost	168,513	119,602
Fair Value
Fair Value	168,220	119,936
Fixed maturities \| Commercial mortgage-backed securities
Amortized Cost
Amortized Cost	456,095	367,848
Fair Value
Fair Value	492,958	382,916
Fixed maturities \| Residential mortgage-backed securities
Amortized Cost
Amortized Cost	54,680	60,778
Fair Value
Fair Value	$ 59,023	$ 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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