Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	$ 780,903	$ 839,474	$ 879,121	$ 825,149
Change in accumulated other comprehensive income (loss) before reclassifications	(55,305)	(12,636)	(168,148)	(27,062)
Reclassification adjustments from accumulated other comprehensive income (loss)	1,563	1,087	3,301	4,230
Balance, ending of year	700,795	814,529	700,795	814,529
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(55,505)		49,769
Change in accumulated other comprehensive income (loss) before reclassifications	(51,678)		(157,266)
Reclassification adjustments from accumulated other comprehensive income (loss)	853		1,167
Balance, ending of year	(106,330)		(106,330)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(1,263)		4,568
Change in accumulated other comprehensive income (loss) before reclassifications	(3,627)		(10,882)
Reclassification adjustments from accumulated other comprehensive income (loss)	710		2,134
Balance, ending of year	(4,180)		(4,180)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(56,768)	55,628	54,337	66,911
Balance, ending of year	$ (110,510)	$ 44,079	$ (110,510)	$ 44,079