Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 151,032
Due after one year through five years	1,188,945
Due after five years through ten years	1,542,382
Due after ten years	7,887,911
Amortized Cost	11,857,110	$ 10,186,465
Fair Value
Due in one year or less	152,032
Due after one year through five years	1,217,111
Due after five years through ten years	1,628,079
Due after ten years	8,228,710
Fair Value	12,338,984	$ 9,771,673
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	479,696
Fair Value
Debt Maturities, without Single Maturity Date	482,130
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	526,795
Fair Value
Debt Maturities, without Single Maturity Date	546,504
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	80,349
Fair Value
Debt Maturities, without Single Maturity Date	$ 84,418

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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