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	$ 139,614
Due after one year through five years	688,411
Due after five years through ten years	1,073,731
Due after ten years	2,833,282
Amortized Cost	5,283,266	$ 5,244,903
Fair Value
Due in one year or less	140,916
Due after one year through five years	697,057
Due after five years through ten years	1,115,368
Due after ten years	3,161,973
Fair Value	5,681,970	$ 5,199,595
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	119,602
Fair Value
Debt Maturities, without Single Maturity Date	119,936
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	367,848
Fair Value
Debt Maturities, without Single Maturity Date	382,916
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	60,778
Fair Value
Debt Maturities, without Single Maturity Date	$ 63,804

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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