Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	$ 879,121	$ 825,149
Change in accumulated other comprehensive income (loss) before reclassifications	(69,169)	(18,954)
Reclassification adjustments from accumulated other comprehensive income (loss)	460	1,954
Balance, ending of year	835,625	823,599
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	49,769
Change in accumulated other comprehensive income (loss) before reclassifications	(65,542)
Reclassification adjustments from accumulated other comprehensive income (loss)	(251)
Balance, ending of year	(16,024)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	4,568
Change in accumulated other comprehensive income (loss) before reclassifications	(3,627)
Reclassification adjustments from accumulated other comprehensive income (loss)	711
Balance, ending of year	1,652
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	54,337	66,911
Balance, ending of year	$ (14,372)	$ 49,911