Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 751,795	$ 835,625	$ 740,114	$ 879,121
Change in accumulated other comprehensive income (loss) before reclassifications	(16,143)	(43,673)	(2,568)	(112,842)
Reclassification adjustments from accumulated other comprehensive income (loss)	161	1,277	629	1,737
Ending balance	676,365	780,903	676,365	780,903
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(73,719)		(88,369)
Change in accumulated other comprehensive income (loss) before reclassifications	(15,496)		(1,273)
Reclassification adjustments from accumulated other comprehensive income (loss)	120		547
Ending balance	(89,095)		(89,095)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	266		873
Change in accumulated other comprehensive income (loss) before reclassifications	(647)		(1,295)
Reclassification adjustments from accumulated other comprehensive income (loss)	41		82
Ending balance	(340)		(340)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(73,453)	(14,372)	(87,496)	54,337
Ending balance	$ (89,435)	$ (56,768)	$ (89,435)	$ (56,768)