Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Current income taxes (benefits) [Abstract]
Federal	$ 2,643,000	$ 3,813,000	$ 346,000
State	940,000	1,383,000	98,000
Deferred income taxes (benefits) [Abstract]
Federal	1,413,000	390,000	3,132,000
State	290,000	(203,000)	1,466,000
Total income tax provision	5,286,000	5,383,000	5,042,000
Tax liabilities [Abstract]
Depreciation and fixed assets	33,595,000	31,307,000
Intangible assets	9,104,000	9,092,000
Other	44,000	15,000
Gross deferred tax liability	42,743,000	40,414,000
Tax Assets [Abstract]
Deferred compensation and post-retirement benefits	5,451,000	5,480,000
Receivables and inventories	315,000	242,000
Accrued liabilities	1,778,000	1,146,000
Derivatives	568,000	968,000
State net operating loss	1,753,000	1,754,000
Other	20,000	20,000
Gross deferred tax asset	9,885,000	9,610,000
Valuation allowance	(1,585,000)	(1,574,000)
Net deferred tax liability	34,443,000	32,378,000
Current deferred tax asset	2,660,000	1,887,000
Net non-current deferred tax liability	37,103,000	34,265,000
Reconciliation of the U.S. income tax rate to the effective income tax rate [Abstract]
Statutory tax rate (in hundredths)	35.00%	35.00%	35.00%
Effect of [Abstract]
State income taxes net of federal tax benefit (in hundredths)	6.20%	6.40%	6.10%
Release of income tax reserve and prior year adjustments (in hundredths)	0.00%	(0.80%)	(2.60%)
Medicare part D subsidy (in hundredths)	0.00%	(0.10%)	0.00%
Uncertain tax positions (in hundredths)	0.20%	0.10%	0.10%
Other, net (in hundredths)	(0.80%)	(1.30%)	(1.10%)
Effective income tax rate (in hundredths)	40.60%	39.30%	37.50%
Unrecognized tax benefits (net of tax) excluding interest	240,000
Unrecognized tax benefits that would impact effective tax rate	213,000
Previous unrecognized tax benefit	13,000
Unrecognized tax benefits, interest on income tax expense	0
Unrecognized Tax Benefits Period Decrease in Next Twelve Months	0
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	12,000	14,000
Roll-forward of unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits opening balance (excluding interest)	314,000	281,000
Increases [Abstract]
Tax positions taken in current period	22,000	18,000
Tax position taken in prior period	0	32,000
Decreases [Abstract]
Settlements	(1,000)	(3,000)
Lapse of statute limitations	(12,000)	(14,000)
Ending balance (excluding interest)	323,000	314,000	281,000
Iowa [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carry-forwards	26,300,000
Minnesota [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carry-forwards	$ 3,400,000
Expiration date	Dec. 31, 2018
Minimum [Member] | Iowa [Member]
Operating Loss Carryforwards [Line Items]
Expiration date	Jan. 01, 2018
Maximum [Member] | Iowa [Member]
Operating Loss Carryforwards [Line Items]
Expiration date	Dec. 31, 2029