Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 733,745	$ 740,114
Change in accumulated other comprehensive income (loss) before reclassifications	(9,361)	13,575
Reclassification adjustments from accumulated other comprehensive income (loss)	2,026	468
Ending balance	736,766	751,795
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(66,967)
Change in accumulated other comprehensive income (loss) before reclassifications	(8,766)
Reclassification adjustments from accumulated other comprehensive income (loss)	2,026
Ending balance	(73,707)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	16,010
Change in accumulated other comprehensive income (loss) before reclassifications	(572)
Reclassification adjustments from accumulated other comprehensive income (loss)	0
Ending balance	15,438
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0
Change in accumulated other comprehensive income (loss) before reclassifications	(23)
Reclassification adjustments from accumulated other comprehensive income (loss)	0
Ending balance	(23)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(50,957)	(87,496)
Ending balance	$ (58,292)	$ (73,453)