Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 206,941
Due after one year through five years	705,933
Due after five years through ten years	993,523
Due after ten years	2,980,483
Amortized Cost	5,468,161	$ 5,244,903
Fair Value
Due in one year or less	208,192
Due after one year through five years	703,752
Due after five years through ten years	1,008,175
Due after ten years	3,096,919
Fair Value	5,606,209	$ 5,199,595
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	148,802
Fair Value
Debt Maturities, without Single Maturity Date	149,733
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	345,909
Fair Value
Debt Maturities, without Single Maturity Date	351,769
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	86,570
Fair Value
Debt Maturities, without Single Maturity Date	$ 87,669

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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