Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	$ 825,149	$ 910,472	$ 888,375
Change in accumulated other comprehensive income before reclassifications	(16,008)	51,753	38,869
Reclassification adjustments from accumulated other comprehensive income	3,434	2,005	4,755
Balance, end of year	879,121	825,149	910,472
Net unrealized appreciation on investments
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	83,070	47,279	(9,323)
Change in accumulated other comprehensive income before reclassifications	(31,519)	37,173	55,399
Reclassification adjustments from accumulated other comprehensive income	(1,782)	(1,382)	1,203
Balance, end of year	49,769	83,070	47,279
Liability for underfunded employee benefit costs
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	(16,159)	(34,127)	(21,149)
Change in accumulated other comprehensive income before reclassifications	15,511	14,580	(16,530)
Reclassification adjustments from accumulated other comprehensive income	5,216	3,387	3,552
Balance, end of year	4,568	(16,159)	(34,127)
Accumulated other comprehensive income, net of tax
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	66,911	13,152	(30,472)
Balance, end of year	$ 54,337	$ 66,911	$ 13,152