Investments (Trading Account Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Trading account assets, at fair value	$ 56,281	$ 35,328
Trading account assets, at amortized cost or cost	50,219	35,484
Fixed Maturities
Investment [Line Items]
Trading account assets, at fair value	38,793	19,558
Trading account assets, at amortized cost or cost	38,279	23,555
Equity securities
Investment [Line Items]
Trading account assets, at fair value	17,488	15,770
Trading account assets, at amortized cost or cost	$ 11,940	$ 11,929