Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized Cost:
Due in one or less	$ 234,713
Due after one year through five years	1,202,685
Due after five years through ten years	2,187,012
Due after ten years	9,550,307
Amortized Cost	14,517,513	$ 12,465,746
Fair Value
Due in one year or less	237,640
Due after one year through five years	1,243,281
Due after five years through ten years	2,301,245
Due after ten years	11,713,249
Fair Value	16,894,879	13,202,365
Asset-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	621,254
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	619,143
Commercial mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	647,722
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	700,435
Residential mortgage-backed securities
Amortized Cost:
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	73,820
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	79,886
Fixed maturities
Amortized Cost:
Amortized Cost	14,517,513	12,465,746
Fair Value
Fair Value	16,894,879	13,202,365
Fixed maturities \| Asset-backed securities
Amortized Cost:
Amortized Cost	621,254	443,767
Fair Value
Fair Value	619,143	444,438
Fixed maturities \| Commercial mortgage-backed securities
Amortized Cost:
Amortized Cost	647,722	557,584
Fair Value
Fair Value	700,435	578,289
Fixed maturities \| Residential mortgage-backed securities
Amortized Cost:
Amortized Cost	73,820	73,814
Fair Value
Fair Value	$ 79,886	$ 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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