Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 90,826
Due after one year through five years	1,116,306
Due after five years through ten years	1,762,440
Due after ten years	8,421,009
Amortized Cost	12,465,746	$ 10,186,465
Fair Value
Due in one year or less	91,258
Due after one year through five years	1,149,228
Due after five years through ten years	1,862,864
Due after ten years	8,998,527
Fair Value	13,202,365	9,771,673
Fixed maturities
Amortized Cost
Amortized Cost	12,465,746	10,186,465
Fair Value
Fair Value	13,202,365	9,771,673
Fixed maturities \| Asset-backed securities
Amortized Cost
Amortized Cost	443,767	505,862
Fair Value
Fair Value	444,438	505,244
Fixed maturities \| Commercial mortgage-backed securities
Amortized Cost
Amortized Cost	557,584	364,601
Fair Value
Fair Value	578,289	361,880
Fixed maturities \| Residential mortgage-backed securities
Amortized Cost
Amortized Cost	73,814	48,622
Fair Value
Fair Value	$ 77,761	$ 49,414

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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