Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	$ 878,897
Change in accumulated other comprehensive income before reclassifications	28,519	$ 8,524
Reclassification adjustments from accumulated other comprehensive income (loss)	(179)	$ (18)
Balance, end of period	929,169
Net unrealized appreciation on investments [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	128,369
Change in accumulated other comprehensive income before reclassifications	28,519
Reclassification adjustments from accumulated other comprehensive income (loss)	(1,070)
Balance, end of period	155,818
Liability for underfunded employee benefit costs [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(47,932)
Change in accumulated other comprehensive income before reclassifications	0
Reclassification adjustments from accumulated other comprehensive income (loss)	891
Balance, end of period	(47,041)
Total [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	80,437
Change in accumulated other comprehensive income before reclassifications	28,519
Reclassification adjustments from accumulated other comprehensive income (loss)	(179)
Balance, end of period	$ 108,777