Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current income taxes (benefits) [Abstract]
Federal	$ 3,813,000	$ 346,000	$ (936,000)
State	1,383,000	98,000	(266,000)
Deferred income taxes (benefits) [Abstract]
Federal	390,000	3,132,000	5,428,000
State	(203,000)	1,466,000	139,000
Total income tax provision	5,383,000	5,042,000	4,365,000
Tax liabilities [Abstract]
Depreciation and fixed assets	31,307,000	30,142,000
Intangible assets	9,092,000	8,800,000
Other	15,000	201,000
Gross deferred tax liability	40,414,000	39,143,000
Tax Assets [Abstract]
Deferred compensation and post-retirement benefits	5,480,000	7,922,000
Receivables and inventories	242,000	311,000
Accrued liabilities	1,146,000	814,000
Derivatives	968,000	983,000
State net operating loss	1,754,000	1,759,000
Other	20,000	19,000
Gross deferred tax asset	9,610,000	11,808,000
Valuation allowance	(1,574,000)	(1,733,000)
Net deferred tax liability	32,378,000	29,068,000
Current deferred tax asset	1,887,000	1,559,000
Net non-current deferred tax liability	34,265,000	30,627,000
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)	6.40%	6.10%	6.10%
Release of income tax reserve and prior Year adjustments (in hundredths)	(0.80%)	(2.60%)	(16.30%)
Medicare part D subsidy (in hundredths)	(0.10%)	0.00%	1.60%
Uncertain tax positions (in hundredths)	0.10%	0.10%	0.40%
Other, net (in hundredths)	(1.30%)	(1.10%)	(1.00%)
Effective income tax rate (in hundredths)	39.30%	37.50%	25.80%
Unrecognized tax benefits (net of tax) excluding interest	232,000
Unrecognized tax benefits that would impact effective tax rate	207,000
Previous unrecognized tax benefit	13,000
Unrecognized tax benefits, interest on income tax expense	1,000
Unrecognized tax benefits that may decrease due to expirations of statute of limitations	0
Roll-forward of unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits opening balance (excluding interest)	281,000	697,000
Increases [Abstract]
Tax positions taken in current period	18,000	8,000
Tax position taken in prior period	32,000	0
Settlements	(3,000)	(15,000)
Lapse of statute limitations	(14,000)	(409,000)
Ending balance (excluding interest)	314,000	281,000	697,000
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
Minimum [Member] | Iowa [Member]
Operating Loss Carryforwards [Line Items]
Expiration date	2018
Maximum [Member] | Iowa [Member]
Operating Loss Carryforwards [Line Items]
Expiration date	2029