Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	$ 835,625	$ 823,599	$ 879,121	$ 825,149
Change in accumulated other comprehensive income (loss) before reclassifications	(43,673)	4,529	(112,842)	(14,426)
Reclassification adjustments from accumulated other comprehensive income (loss)	1,277	1,188	1,737	3,143
Balance, ending of year	780,903	839,475	780,903	839,475
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(16,024)		49,769
Change in accumulated other comprehensive income (loss) before reclassifications	(40,046)		(105,588)
Reclassification adjustments from accumulated other comprehensive income (loss)	565		314
Balance, ending of year	(55,505)		(55,505)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	1,652		4,568
Change in accumulated other comprehensive income (loss) before reclassifications	(3,627)		(7,254)
Reclassification adjustments from accumulated other comprehensive income (loss)	712		1,423
Balance, ending of year	(1,263)		(1,263)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(14,372)	49,911	54,337	66,911
Balance, ending of year	$ (56,768)	$ 55,628	$ (56,768)	$ 55,628