Accumulated Other Comprehensive Income (Loss) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	$ 910,472
Change in accumulated other comprehensive income before reclassifications	4,404
Reclassification adjustments from accumulated other comprehensive income (loss)	943
Balance, end of year	833,144
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	47,279
Change in accumulated other comprehensive income before reclassifications	4,404
Reclassification adjustments from accumulated other comprehensive income (loss)	96
Balance, end of year	51,779
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(34,127)
Change in accumulated other comprehensive income before reclassifications	0
Reclassification adjustments from accumulated other comprehensive income (loss)	847
Balance, end of year	(33,280)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	13,152
Balance, end of year	$ 18,499