Acquisition of Gordon Trucking, Inc. Assets acquired and liabilities assumed (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Nov. 11, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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		$ 119,838	$ 139,770	$ 121,120
Other current assets	11,061		2,281
Property, Plant and Equipment, Gross	622,864		432,330
Other Assets, Noncurrent	13,850		9,110
Finite-Lived Intangible Assets, Gross	19,042
Goodwill	98,686		4,815	4,815
Assets	724,841		467,737
Long-term Debt	75,000
Other long-term liabilities	13,618		0
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