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 or less	$ 202,191
Due after one year through five years	1,184,697
Due after five years through ten years	2,323,234
Due after ten years	9,677,499
Amortized Cost	14,916,971	$ 12,465,746
Fair Value
Due in one year or less	204,939
Due after one year through five years	1,244,727
Due after five years through ten years	2,502,344
Due after ten years	11,849,063
Fair Value	17,398,916	13,202,365
Asset-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	804,636
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	809,451
Commercial mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	649,810
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	706,939
Residential mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	74,904
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	81,453
Fixed maturities
Amortized Cost:
Amortized Cost	14,916,971	12,465,746
Fair Value
Fair Value	17,398,916	13,202,365
Fixed maturities \| Asset-backed securities
Amortized Cost:
Amortized Cost	804,636	443,767
Fair Value
Fair Value	809,451	444,438
Fixed maturities \| Commercial mortgage-backed securities
Amortized Cost:
Amortized Cost	649,810	557,584
Fair Value
Fair Value	706,939	578,289
Fixed maturities \| Residential mortgage-backed securities
Amortized Cost:
Amortized Cost	74,904	73,814
Fair Value
Fair Value	$ 81,453	$ 77,761

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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