Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 740,114	$ 879,121
Change in accumulated other comprehensive income (loss) before reclassifications	13,575	(69,169)
Reclassification adjustments from accumulated other comprehensive income (loss)	468	460
Ending balance	751,795	835,625
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(88,369)
Change in accumulated other comprehensive income (loss) before reclassifications	14,223
Reclassification adjustments from accumulated other comprehensive income (loss)	427
Ending balance	(73,719)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	873
Change in accumulated other comprehensive income (loss) before reclassifications	(648)
Reclassification adjustments from accumulated other comprehensive income (loss)	41
Ending balance	266
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(87,496)	54,337
Ending balance	$ (73,453)	$ (14,372)