Employee Benefit Plans (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014 Retiree		Dec. 31, 2013 Retiree		Dec. 31, 2012		Dec. 31, 2015
Amounts Recognized in the Balance Sheet
Long-term liabilities	$ 35,827,000		$ 33,554,000
Postretirement Benefits Other than Pensions
Pension and Other Postretirement Benefits
Number of retirees and their dependents to whom benefits under the plan are provided	278		270
Change in Benefit Obligation
Benefit obligation at beginning of year	34,995,000		40,168,000		39,969,000
Service cost	1,295,000		1,739,000		1,513,000
Interest cost	1,343,000		1,500,000		1,537,000
Actuarial (gain) loss	373,000		(7,618,000)		(2,073,000)
Benefits paid	(930,000)		(794,000)		(778,000)
Benefit obligation at end of year	37,076,000		34,995,000		40,168,000
Change in Plan Assets
Fair value of plan assets at end of year	0		0		0
Funded status at end of year	(37,076,000)		(34,995,000)		(40,168,000)
Amounts Recognized in the Balance Sheet
Current liabilities	1,249,000		1,441,000		1,304,000
Long-term liabilities	35,827,000		33,554,000		38,864,000
Amounts recognized in the balance sheet	37,076,000		34,995,000		40,168,000
Amounts Recognized in Accumulated Other Comprehensive Income
Net actuarial loss	(3,408,000)		(3,010,000)		(11,269,000)
Amount recognized to accumulated other comprehensive income, net actuarial loss	3,408,000		3,010,000		11,269,000
Components of Net Periodic Postretirement Benefit Cost
Service cost	1,295,000		1,739,000		1,513,000
Interest cost	1,343,000		1,500,000		1,537,000
Amortization of net (gain) loss	(26,000)		641,000		824,000
Net periodic postretirement cost	2,612,000		3,880,000		3,874,000
Other Changes in Benefit Obligations Recognized in Other Comprehensive Income (Loss)
Net (gain) loss	373,000		(7,618,000)		(2,073,000)
Amortization of net (gain) loss	26,000		(641,000)		(824,000)
Total recognized in other comprehensive income (loss)	399,000		(8,259,000)		(2,897,000)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	3,011,000		(4,379,000)		977,000
Weighted-average assumptions used to determine projected pension benefit obligations
Discount rate (as a percent)	4.00%	[1]	4.75%	[1]	4.00%	[1]
Health care cost trend rate for medical benefits assumed for next year (as a percent)	6.00%		6.50%		7.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) (as a percent)	4.50%		4.50%		5.00%
Year that the rate reaches the ultimate trend rate	2018		2018		2015
Discount rate beginning of year (as a percent)	4.75%		4.00%		4.25%
Percentage of the expected annual fully-insured premium by which subsidy is limited for participants age 65 and older	60.00%
Subsidy limit which should not exceed aggregate annual amount for participants under age 65	648,000
Annual increase in subsidy limit (as a percent), for participants under age 65	3.50%
Postretirement Benefits Other than Pensions | Forecast
Change in Benefit Obligation
Interest cost							1,300,000
Components of Net Periodic Postretirement Benefit Cost
Interest cost							$ 1,300,000

[1] Represents the year end rates used to determine the projected benefit obligation. To compute postretirement cost in 2014, 2013 and 2012, respectively, the beginning of year discount rates of 4.75%, 4.00% and 4.25% were used.