Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016		Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year				$ 878,897
Change in accumulated other comprehensive income before reclassifications		$ (6,136)	$ (1,305)	54,448		$ (16,336)
Reclassification adjustments from accumulated other comprehensive income (loss)		(617)	$ 27	(359)		$ 576
Balance, end of period		959,180		959,180
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year		187,428		128,369
Change in accumulated other comprehensive income before reclassifications		(6,136)		54,448
Reclassification adjustments from accumulated other comprehensive income (loss)		(1,508)		(3,033)
Balance, end of period		179,784		179,784
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year		(46,149)		(47,932)	[1]
Change in accumulated other comprehensive income before reclassifications	[1]	0		0
Reclassification adjustments from accumulated other comprehensive income (loss)	[1]	891		2,674
Balance, end of period	[1]	(45,258)		(45,258)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year		141,279		80,437
Change in accumulated other comprehensive income before reclassifications		(6,136)		54,448
Reclassification adjustments from accumulated other comprehensive income (loss)		(617)		(359)
Balance, end of period		$ 134,526		$ 134,526

[1] Estimates and Assumptions: The preparation of financial statements in conformity with GAAP requires us to make various estimates and assumptions that affect the reporting of net periodic benefit cost, plan assets and plan obligations for each plan at the date of the financial statements. Actual results could differ from these estimates. One significant estimate relates to the calculation of the benefit obligation for each plan. We annually establish the discount rate, which is an estimate of the interest rate at which these benefits could be effectively settled, that is used to determine the present value of the respective plan's benefit obligations as of December 31.