Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 736,766	$ 751,795	$ 733,745	$ 740,114
Change in accumulated other comprehensive income (loss) before reclassifications	(5,794)	(16,143)	(15,155)	(2,568)
Reclassification adjustments from accumulated other comprehensive income (loss)	970	161	2,996	629
Ending balance	726,595	676,366	726,595	676,366
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(58,292)	(73,453)	(50,957)	(87,496)
Ending balance	(63,116)	$ (89,435)	(63,116)	$ (89,435)
Net unrealized gain (loss) on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(73,707)		(66,967)
Change in accumulated other comprehensive income (loss) before reclassifications	(5,257)		(14,023)
Reclassification adjustments from accumulated other comprehensive income (loss)	970		2,996
Ending balance	(77,994)		(77,994)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	15,438		16,010
Change in accumulated other comprehensive income (loss) before reclassifications	(572)		(1,144)
Reclassification adjustments from accumulated other comprehensive income (loss)	0		0
Ending balance	14,866		14,866
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(23)		0
Change in accumulated other comprehensive income (loss) before reclassifications	35		12
Reclassification adjustments from accumulated other comprehensive income (loss)	0		0
Ending balance	$ 12		$ 12