Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 676,365	$ 780,903	$ 740,114	$ 879,121
Change in accumulated other comprehensive income (loss) before reclassifications	(34,812)	(55,305)	(37,380)	(168,148)
Reclassification adjustments from accumulated other comprehensive income (loss)	102	1,563	731	3,301
Ending balance	644,937	700,795	644,937	700,795
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(89,094)		(88,369)
Change in accumulated other comprehensive income (loss) before reclassifications	(34,166)		(35,438)
Reclassification adjustments from accumulated other comprehensive income (loss)	62		609
Ending balance	(123,198)		(123,198)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(341)		873
Change in accumulated other comprehensive income (loss) before reclassifications	(646)		(1,942)
Reclassification adjustments from accumulated other comprehensive income (loss)	40		122
Ending balance	(947)		(947)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(89,435)	(56,768)	(87,496)	54,337
Ending balance	$ (124,145)	$ (110,510)	$ (124,145)	$ (110,510)