Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 955,191	$ 888,375
Change in accumulated other comprehensive income before reclassifications	22,592	48,941
Reclassification adjustments from accumulated other comprehensive income (loss)	858	1,676
Ending Balance	968,303	968,303
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	16,956	(9,323)
Change in accumulated other comprehensive income before reclassifications	22,592	48,941
Reclassification adjustments from accumulated other comprehensive income (loss)	(30)	(100)
Ending Balance	39,518	39,518
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(20,261)	(21,149)
Change in accumulated other comprehensive income before reclassifications	0	0
Reclassification adjustments from accumulated other comprehensive income (loss)	888	1,776
Ending Balance	(19,373)	(19,373)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(3,305)	(30,472)
Ending Balance	$ 20,145	$ 20,145