Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	$ 833,144	$ 910,472
Change in accumulated other comprehensive income before reclassifications	27,566	31,971
Reclassification adjustments from accumulated other comprehensive income (loss)	785	1,727
Balance, ending	860,667	860,667
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	51,779	47,279
Change in accumulated other comprehensive income before reclassifications	27,566	31,971
Reclassification adjustments from accumulated other comprehensive income (loss)	(62)	33
Balance, ending	79,283	79,283
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	(33,280)	(34,127)
Change in accumulated other comprehensive income before reclassifications	0	0
Reclassification adjustments from accumulated other comprehensive income (loss)	847	1,694
Balance, ending	(32,433)	(32,433)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	18,499	13,152
Balance, ending	$ 46,850	$ 46,850