Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 149,506
Due after one year through five years	710,210
Due after five years through ten years	1,053,548
Due after ten years	2,923,882
Amortized Cost	5,418,988	$ 5,244,903
Fair Value
Due in one year or less	150,990
Due after one year through five years	713,547
Due after five years through ten years	1,089,987
Due after ten years	3,291,608
Fair Value	5,856,022	$ 5,199,595
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	124,717
Fair Value
Debt Maturities, without Single Maturity Date	125,461
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	391,828
Fair Value
Debt Maturities, without Single Maturity Date	415,992
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	65,297
Fair Value
Debt Maturities, without Single Maturity Date	$ 68,437

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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