Investments, Net Realized and Unrealized Gains (Losses) on Investments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Components of net realized and unrealized gains (losses) on investments [Abstract]
Gross gains on available-for-sale fixed income securities sold during the period	$ 2,407	$ 2,690	$ 9,710	$ 8,824
Gross losses on available-for-sale fixed income securities sold during the period	(2,728)	(2,743)	(9,409)	(8,539)
Other-than-temporary impairments	(58)	0	(404)	0
Change in value of limited partnership investments	278	(1,073)	1,000	(6,518)
Gains on equity securities [Abstract]
Realized gains on equity securities sold during the period	1,520	502	1,571	1,455
Unrealized gains (losses) on equity securities held at the end of the period	(1,294)	2,997	6,573	(817)
Realized and unrealized gains on equity securities during the period	226	3,499	8,144	638
Net realized and unrealized gains (losses) on investments	$ 125	$ 2,373	9,041	$ (5,595)
Federal income taxes			$ 16,035