Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost:
Due in one or less	$ 303,632
Due after one year through five years	2,815,832
Due after five years through ten years	2,566,636
Due after ten years	3,615,942
Amortized Cost	11,178,085	$ 16,177,891
Fair Value
Due in one year or less	307,998
Due after one year through five years	2,890,358
Due after five years through ten years	2,671,469
Due after ten years	3,741,417
Fair Value	11,525,964	18,584,685
Asset-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	975,298
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	984,988
Commercial mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	835,555
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	859,942
Residential mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	65,190
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	69,792
Fixed maturities
Amortized Cost:
Amortized Cost	11,178,085	16,177,891
Fair Value
Fair Value	11,525,964	18,584,685
Fixed maturities \| Asset-backed securities
Amortized Cost:
Amortized Cost	975,298	984,318
Fair Value
Fair Value	984,988	992,583
Fixed maturities \| Commercial mortgage-backed securities
Amortized Cost:
Amortized Cost	835,555	728,522
Fair Value
Fair Value	859,942	785,942
Fixed maturities \| Residential mortgage-backed securities
Amortized Cost:
Amortized Cost	65,190	69,909
Fair Value
Fair Value	$ 69,792	$ 75,724

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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