Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost:
Due in one or less	$ 264,241
Due after one year through five years	2,979,491
Due after five years through ten years	3,386,537
Due after ten years	3,757,756
Amortized Cost	12,587,739	$ 16,177,891
Fair Value
Due in one year or less	270,212
Due after one year through five years	3,083,315
Due after five years through ten years	3,544,241
Due after ten years	4,076,803
Fair Value	13,222,809	18,584,685
Asset-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	1,306,017
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	1,313,052
Commercial mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	823,769
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	861,117
Residential mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	69,928
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	74,069
Fixed maturities
Amortized Cost:
Amortized Cost	12,587,739	16,177,891
Fair Value
Fair Value	13,222,809	18,584,685
Fixed maturities \| Asset-backed securities
Amortized Cost:
Amortized Cost	1,306,017	984,318
Fair Value
Fair Value	1,313,052	992,583
Fixed maturities \| Commercial mortgage-backed securities
Amortized Cost:
Amortized Cost	823,769	728,522
Fair Value
Fair Value	861,117	785,942
Fixed maturities \| Residential mortgage-backed securities
Amortized Cost:
Amortized Cost	69,928	69,909
Fair Value
Fair Value	$ 74,069	$ 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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