Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 107,422
Due after one year through five years	714,425
Due after five years through ten years	1,034,940
Due after ten years	2,839,618
Amortized Cost	5,246,007	$ 4,941,944
Fair Value
Due in one year or less	108,362
Due after one year through five years	706,440
Due after five years through ten years	1,026,399
Due after ten years	2,848,458
Fair Value	5,234,708	$ 5,223,302
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	162,353
Fair Value
Debt Maturities, without Single Maturity Date	164,015
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	295,265
Fair Value
Debt Maturities, without Single Maturity Date	289,658
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	91,984
Fair Value
Debt Maturities, without Single Maturity Date	$ 91,376

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
-Publisher FASB
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