Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	$ 879,121	$ 825,149	$ 910,472
Change in accumulated other comprehensive income before reclassifications	(145,723)	(16,008)	51,753
Reclassification adjustments from accumulated other comprehensive income	3,890	3,434	2,005
Balance, end of year	740,114	879,121	825,149
Net unrealized appreciation on investments
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	49,769	83,070	47,279
Change in accumulated other comprehensive income before reclassifications	(139,183)	(31,519)	37,173
Reclassification adjustments from accumulated other comprehensive income	1,045	(1,782)	(1,382)
Balance, end of year	(88,369)	49,769	83,070
Liability for underfunded employee benefit costs
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	4,568	(16,159)	(34,127)
Change in accumulated other comprehensive income before reclassifications	(6,540)	15,511	14,580
Reclassification adjustments from accumulated other comprehensive income	2,845	5,216	3,387
Balance, end of year	873	4,568	(16,159)
Accumulated other comprehensive income, net of tax
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	54,337	66,911	13,152
Balance, end of year	$ (87,496)	$ 54,337	$ 66,911