Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 237,808
Due after one year through five years	1,251,310
Due after five years through ten years	1,410,333
Due after ten years	5,704,928
Amortized Cost	9,745,836	$ 10,145,266
Fair Value
Due in one year or less	239,220
Due after one year through five years	1,250,232
Due after five years through ten years	1,419,452
Due after ten years	5,347,349
Fair Value	9,386,801	$ 10,110,786
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	543,879
Fair Value
Debt Maturities, without Single Maturity Date	546,435
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	503,358
Fair Value
Debt Maturities, without Single Maturity Date	490,931
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	94,220
Fair Value
Debt Maturities, without Single Maturity Date	$ 93,182

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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