Investments - Realized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Net realized investment gains (losses)	$ (3,406)	$ 4,123	$ 12,713
Fixed maturity securities
Gain (Loss) on Investments [Line Items]
Net realized investment gains (losses)	(8,867)	5,784	10,289
Equity securities
Gain (Loss) on Investments [Line Items]
Net realized investment gains (losses)	4,003	(608)	1,378
Short-term and other investments
Gain (Loss) on Investments [Line Items]
Net realized investment gains (losses)	$ 1,458	$ (1,053)	$ 1,046