Summary of Significant Accounting Policies - Assets and liabilities measured at fair value on recurring basis (Details 4) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Level 1
Assets:
Total Assets	$ 423	$ 2,092
Liabilities:
Total Liabilities
Level 1 | Forward Energy Contracts
Assets:
Derivative Assets
Liabilities:
Derivative Liabilities:
Level 1 | Money Market Fund
Assets:
Other Current Assets - Escrow Account IPH Sale		1,500
Level 1 | Money Market and Mutual Funds
Assets:
Other Current Assets - Nonqualified Retirement Savings Plan	110	110
Other Assets - Nonqualified Retirement Savings Plan	183	357
Level 1 | Equity Securities
Assets:
Other Assets - Nonqualified Retirement Savings Plan	130	125
Level 2
Assets:
Total Assets	8,989	9,353
Liabilities:
Total Liabilities		242
Level 2 | Forward Energy Contracts
Assets:
Derivative Assets		292
Liabilities:
Derivative Liabilities:		242
Level 2 | Forward Gasoline Purchase Contracts
Assets:
Derivative Assets	94	136
Level 2 | Corporate Debt Securities
Assets:
Investments - Held by Captive Insurance Company	7,617	7,620
Level 2 | U.S. Government Debt Securities
Assets:
Investments - Held by Captive Insurance Company	1,278	1,305
Level 3
Assets:
Total Assets	1,180	210
Liabilities:
Total Liabilities	13,294	17,992
Level 3 | Forward Energy Contracts
Assets:
Derivative Assets	1,180	210
Liabilities:
Derivative Liabilities:	$ 13,294	$ 17,992