Acquisition of Gordon Trucking, Inc. Assets acquired and liabilities assumed (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 USD ($)	Nov. 11, 2013 USD ($)	Dec. 31, 2012 USD ($)
Business Acquisition [Line Items]
Assets of business combination as percentage total assets	0.486
Business Acquisition, Percentage of Voting Interests Acquired				100.00%
Cash and cash equivalents	$ 17,303	$ 17,763	$ 33,232		$ 119,838
Other current assets	8,992		8,316
Property, Plant and Equipment, Gross	678,566		671,946
Other Assets, Noncurrent	12,611		11,363
Finite-Lived Intangible Assets, Gross	19,042		19,042
Goodwill	100,212	98,686	100,212
Assets	759,994		736,030
Other long-term liabilities	$ (11,318)		$ (12,153)
Revenues of business combination as percentage of total revenues	0.096
Gordon Trucking Company, Inc [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				$ 21,485
Accounts Receivable, Net				45,679
Other current assets				14,371
Property, Plant and Equipment, Gross				189,409
Other Assets, Noncurrent				3,916
Finite-Lived Intangible Assets, Gross				19,042
Goodwill				95,371
Assets				389,273
Accounts Payable and Accrued Liabilities				(30,665)
Self Insurance Reserve				(23,821)
Long-term Debt				(147,942)
Business Combination, Contingent Consideration, Liability				(13,618)
Other long-term liabilities				$ (1,227)
Total consideration transferred to acquire a business		$ 172,000