Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year				$ 878,897
Change in accumulated other comprehensive income before reclassifications		$ 32,065	$ (23,555)	60,584	$ (15,031)
Reclassification adjustments from accumulated other comprehensive income (loss)		437	$ 567	258	$ 549
Balance, end of period		961,031		961,031
Net unrealized appreciation on investments [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year		155,818		128,369
Change in accumulated other comprehensive income before reclassifications		32,065		60,584
Reclassification adjustments from accumulated other comprehensive income (loss)		(455)		(1,525)
Balance, end of period		187,428		187,428
Liability for underfunded employee benefit costs [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	[1]	(47,041)		(47,932)
Change in accumulated other comprehensive income before reclassifications	[1]	0		0
Reclassification adjustments from accumulated other comprehensive income (loss)	[1]	892		1,783
Balance, end of period	[1]	(46,149)		(46,149)
Total [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year		108,777		80,437
Change in accumulated other comprehensive income before reclassifications		32,065		60,584
Reclassification adjustments from accumulated other comprehensive income (loss)		437		258
Balance, end of period		$ 141,279		$ 141,279

[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.