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	$ 191,944
Due after one year through five years	1,198,681
Due after five years through ten years	1,440,920
Due after ten years	7,896,937
Amortized Cost	11,736,191	$ 10,186,465
Fair Value
Due in one year or less	192,769
Due after one year through five years	1,210,583
Due after five years through ten years	1,477,358
Due after ten years	7,804,565
Fair Value	11,700,494	$ 9,771,673
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	512,855
Fair Value
Debt Maturities, without Single Maturity Date	512,969
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	411,390
Fair Value
Debt Maturities, without Single Maturity Date	416,733
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	83,464
Fair Value
Debt Maturities, without Single Maturity Date	$ 85,517

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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