Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	$ 941,884
Change in accumulated other comprehensive income before reclassifications	9,728	$ 28,519
Reclassification adjustments from accumulated other comprehensive income (loss)	(1,335)	$ (179)
Balance, end of period	960,385
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	133,892
Change in accumulated other comprehensive income before reclassifications	9,728
Reclassification adjustments from accumulated other comprehensive income (loss)	(2,213)
Balance, end of period	141,407
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(24,837)
Change in accumulated other comprehensive income before reclassifications	0
Reclassification adjustments from accumulated other comprehensive income (loss)	878
Balance, end of period	(23,959)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	109,055
Change in accumulated other comprehensive income before reclassifications	9,728
Reclassification adjustments from accumulated other comprehensive income (loss)	(1,335)
Balance, end of period	$ 117,448