Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 726,595	$ 676,366	$ 733,745	$ 740,114
Change in accumulated other comprehensive income (loss) before reclassifications	41,160	(34,812)	26,005	(37,380)
Reclassification adjustments from accumulated other comprehensive income (loss)	1,328	102	4,324	731
Ending balance	785,823	644,937	785,823	644,937
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(63,116)	(89,435)	(50,957)	(87,496)
Ending balance	(20,628)	$ (124,145)	(20,628)	$ (124,145)
Net unrealized gain (loss) on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(77,994)		(66,967)
Change in accumulated other comprehensive income (loss) before reclassifications	40,879		26,856
Reclassification adjustments from accumulated other comprehensive income (loss)	1,328		4,324
Ending balance	(35,787)		(35,787)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	14,866		16,010
Change in accumulated other comprehensive income (loss) before reclassifications	(572)		(1,716)
Reclassification adjustments from accumulated other comprehensive income (loss)	0		0
Ending balance	14,294		14,294
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	12		0
Change in accumulated other comprehensive income (loss) before reclassifications	853		865
Reclassification adjustments from accumulated other comprehensive income (loss)	0		0
Ending balance	$ 865		$ 865