Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year		$ 941,884
Change in accumulated other comprehensive income before reclassifications	$ 12,347	41,999
Reclassification adjustments from accumulated other comprehensive income (loss)	606	(1,133)
Balance, end of period	944,048	944,048
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	160,048	133,892
Change in accumulated other comprehensive income before reclassifications	12,347	41,999
Reclassification adjustments from accumulated other comprehensive income (loss)	(273)	(3,769)
Balance, end of period	172,122	172,122
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(23,080)	(24,837)
Change in accumulated other comprehensive income before reclassifications	0	0
Reclassification adjustments from accumulated other comprehensive income (loss)	879	2,636
Balance, end of period	(22,201)	(22,201)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	136,968	109,055
Balance, end of period	$ 149,921	$ 149,921