Acquisition of Gordon Trucking, Inc. Assets acquired and liabilities assumed (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Nov. 11, 2013	Dec. 31, 2012	Dec. 31, 2011
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	$ 17,763	$ 17,303		$ 119,838	$ 139,770
Other current assets	11,061	8,992
Property, Plant and Equipment, Gross	622,864	678,566
Other Assets, Noncurrent	13,850	12,611
Finite-Lived Intangible Assets, Gross	19,042	19,042
Goodwill	98,686	100,212		4,815
Assets	724,841	759,994
Other long-term liabilities	13,618	11,318
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			93,871
Assets			387,773
Accounts Payable and Accrued Liabilities			(29,165)
Self Insurance Reserve			(23,821)
Long-term Debt			(147,942)
Other long-term liabilities			(14,845)
Total consideration transferred to acquire a business	$ 172,000