Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year			$ 941,884
Change in accumulated other comprehensive income before reclassifications	$ 19,924	$ 32,065	29,652	$ 60,584
Reclassification adjustments from accumulated other comprehensive income (loss)	(404)	$ 437	(1,739)	$ 258
Balance, end of period	962,588		962,588
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	141,407		133,892
Change in accumulated other comprehensive income before reclassifications	19,924		29,652
Reclassification adjustments from accumulated other comprehensive income (loss)	(1,283)		(3,496)
Balance, end of period	160,048		160,048
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(23,959)		(24,837)
Change in accumulated other comprehensive income before reclassifications	0		0
Reclassification adjustments from accumulated other comprehensive income (loss)	879		1,757
Balance, end of period	(23,080)		(23,080)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	117,448		109,055
Change in accumulated other comprehensive income before reclassifications	19,924		29,652
Reclassification adjustments from accumulated other comprehensive income (loss)	(404)		(1,739)
Balance, end of period	$ 136,968		$ 136,968