Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments [Abstract]
Gross realized gains	$ 12,352	$ 28,530	$ 35,035
Gross realized losses	(11,023)	(11,397)	(16,093)
Investment income	1,329	17,133	18,942
Investment impairment	8,300	3,100
Reclassification adjustment for gains included in net income	$ 6,118	$ (18,473)	$ (12,138)