Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized Cost:
Due in one or less	$ 246,536
Due after one year through five years	1,264,902
Due after five years through ten years	1,559,972
Due after ten years	9,045,838
Amortized Cost	13,264,558	$ 12,465,746
Due after one year through five years
Due in one year or less	245,711
Due after one year through five years	1,236,352
Due after five years through ten years	1,518,325
Due after ten years	11,057,536
Fair Value	15,212,476	13,202,365
Asset-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	507,849
Due after one year through five years
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	485,103
Commercial mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	562,409
Due after one year through five years
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	589,518
Residential mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	77,052
Due after one year through five years
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	79,931
Fixed maturities
Amortized Cost:
Amortized Cost	13,264,558	12,465,746
Due after one year through five years
Fair Value	15,212,476	13,202,365
Fixed maturities \| Asset-backed securities
Amortized Cost:
Amortized Cost	507,849	443,767
Due after one year through five years
Fair Value	485,103	444,438
Fixed maturities \| Commercial mortgage-backed securities
Amortized Cost:
Amortized Cost	562,409	557,584
Due after one year through five years
Fair Value	589,518	578,289
Fixed maturities \| Residential mortgage-backed securities
Amortized Cost:
Amortized Cost	77,052	73,814
Due after one year through five years
Fair Value	$ 79,931	$ 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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