Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	$ 839,474	$ 860,667	$ 825,149	$ 910,472
Change in accumulated other comprehensive income before reclassifications	(12,636)	3,252	(27,062)	35,223
Reclassification adjustments from accumulated other comprehensive income (loss)	1,087	805	4,230	2,532
Balance, ending	814,529	820,282	814,529	820,282
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	67,005		83,070
Change in accumulated other comprehensive income before reclassifications	(9,661)		(26,252)
Reclassification adjustments from accumulated other comprehensive income (loss)	(167)		359
Balance, ending	57,177		57,177
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	(11,377)		(16,159)
Change in accumulated other comprehensive income before reclassifications	(2,975)		(810)
Reclassification adjustments from accumulated other comprehensive income (loss)	1,254		3,871
Balance, ending	(13,098)		(13,098)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	55,628	46,850	66,911	13,152
Balance, ending	$ 44,079	$ 50,907	$ 44,079	$ 50,907