Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 968,303	$ 888,375
Change in accumulated other comprehensive income before reclassifications	12,173	61,114
Reclassification adjustments from accumulated other comprehensive income (loss)	785	2,461
Ending Balance	964,168	964,168
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	39,518	(9,323)
Change in accumulated other comprehensive income before reclassifications	12,173	61,114
Reclassification adjustments from accumulated other comprehensive income (loss)	(103)	(203)
Ending Balance	51,588	51,588
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(19,373)	(21,149)
Change in accumulated other comprehensive income before reclassifications	0	0
Reclassification adjustments from accumulated other comprehensive income (loss)	888	2,664
Ending Balance	(18,485)	(18,485)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	20,145	(30,472)
Ending Balance	$ 33,103	$ 33,103