Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	$ 860,667	$ 910,472
Change in accumulated other comprehensive income before reclassifications	3,252	35,223
Reclassification adjustments from accumulated other comprehensive income (loss)	805	2,532
Balance, ending	820,282	820,282
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	79,283	47,279
Change in accumulated other comprehensive income before reclassifications	3,252	35,223
Reclassification adjustments from accumulated other comprehensive income (loss)	(43)	(10)
Balance, ending	82,492	82,492
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	(32,433)	(34,127)
Change in accumulated other comprehensive income before reclassifications	0	0
Reclassification adjustments from accumulated other comprehensive income (loss)	848	2,542
Balance, ending	(31,585)	(31,585)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	46,850	13,152
Balance, ending	$ 50,907	$ 50,907