Investments, Net Realized and Unrealized Gains (Losses) on Investments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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	$ 4,132	$ 3,691	$ 7,303	$ 6,134
Gross losses on available-for-sale fixed income securities sold during the period	(3,154)	(3,088)	(6,681)	(5,796)
Other-than-temporary impairments	(86)	0	(346)	0
Change in value of limited partnership investments	314	(2,842)	722	(5,445)
Gains (losses) on equity securities [Abstract]
Realized gains (losses) on equity securities sold during the period	(265)	312	51	953
Unrealized gains (losses) on equity securities held at the end of the period	1,948	(1,508)	7,867	(3,814)
Realized and unrealized gains (losses) on equity securities during the period	1,683	(1,196)	7,918	(2,861)
Net realized and unrealized gains (losses) on investments	$ 2,889	$ (3,435)	8,916	$ (7,968)
Federal income taxes			$ 25,071