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	$ 128,354
Due after one year through five years	1,107,727
Due after five years through ten years	1,649,659
Due after ten years	8,116,218
Amortized Cost	12,076,220	$ 10,186,465
Fair Value
Due in one year or less	129,099
Due after one year through five years	1,133,628
Due after five years through ten years	1,736,529
Due after ten years	9,062,316
Fair Value	13,172,795	$ 9,771,673
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	443,291
Fair Value
Debt Maturities, without Single Maturity Date	445,456
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	554,117
Fair Value
Debt Maturities, without Single Maturity Date	584,181
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	76,854
Fair Value
Debt Maturities, without Single Maturity Date	$ 81,586

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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