Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	$ 823,599	$ 833,144	$ 825,149	$ 910,472
Change in accumulated other comprehensive income before reclassifications	4,529	27,566	(14,426)	31,971
Reclassification adjustments from accumulated other comprehensive income (loss)	1,188	785	3,143	1,727
Balance, ending	839,475	860,667	839,475	860,667
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	59,614		83,070
Change in accumulated other comprehensive income before reclassifications	7,503		(16,591)
Reclassification adjustments from accumulated other comprehensive income (loss)	(112)		526
Balance, ending	67,005		67,005
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	(9,703)		(16,159)
Change in accumulated other comprehensive income before reclassifications	(2,974)		2,165
Reclassification adjustments from accumulated other comprehensive income (loss)	1,300		2,617
Balance, ending	(11,377)		(11,377)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	49,911	18,499	66,911	13,152
Balance, ending	$ 55,628	$ 46,850	$ 55,628	$ 46,850