Acquisition of Gordon Trucking, Inc. (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2017	Nov. 01, 2016	Dec. 31, 2015	Nov. 01, 2015	Dec. 31, 2014	Nov. 01, 2014	Nov. 11, 2013
Business Acquisition [Line Items]
Total acquisition purchase price	$ 300,000,000
Payments to Acquire Intangible Assets	(15,000,000)
Fair Value of Stock in Escrow Related To Acquisition of Business								24,000,000
Fair Value of Cash in Escrow as a Result of Business Combination								6,000,000
Noncash or Part Noncash Acquisition, Debt Assumed	148,000,000
Line of Credit Facility, Maximum Borrowing Capacity	250,000,000		175,000,000		200,000,000		225,000,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	20,000,000	8,000,000		6,000,000		6,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net								1,500,000
Cash Acquired from Acquisition	(20,000,000)
Business Combination, Acquisition Related Costs	2,200,000
Gordon Trucking Company, Inc [Member]
Business Acquisition [Line Items]
Total acquisition purchase price	$ 285,000,000