Investments - Net Effect on Stockholders' Equity of Unrealized Gains and Losses on AFS Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax [Abstract]
Net unrealized investment gains including OTTI, Fixed-maturity and Equity securities	$ 60,292	$ 65,831
OTTI	174	95
Net unrealized investment gains excluding OTTI	60,466	65,926
Less deferred income taxes	(20,780)	(23,074)
Net unrealized investment gains excluding OTTI, net of tax	$ 39,686	$ 42,852