Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	$ 825,149	$ 910,472
Change in accumulated other comprehensive income before reclassifications	(18,954)	4,404
Reclassification adjustments from accumulated other comprehensive income (loss)	1,954	943
Balance, ending	823,599	833,144
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	83,070
Change in accumulated other comprehensive income before reclassifications	(24,093)
Reclassification adjustments from accumulated other comprehensive income (loss)	637
Balance, ending	59,614
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	(16,159)
Change in accumulated other comprehensive income before reclassifications	5,139
Reclassification adjustments from accumulated other comprehensive income (loss)	1,317
Balance, ending	(9,703)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning	66,911	13,152
Balance, ending	$ 49,911	$ 18,499