Related Party (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Payments to Acquire Property, Plant, and Equipment	$ 184,114	$ 86,088	$ 217,253
Due to Related Parties, Current	100	100
Operating Leases, Rent Expense	8,000	1,000	3,300
Related Party Transaction, Expenses from Transactions with Related Party	2,263	8,154	53,039
Related Party [Member]
Related Party Transaction [Line Items]
Payments to Acquire Property, Plant, and Equipment	0	0	21,555
Payments for tractor purchase	0	4,300	58,599
proceeds from sale of trailers	(12)	(108)	(28)
Operating Leases, Rent Expense		1,000	3,000
proceeds from sale of tractors	0	0	(38,064)
Payments for parts and services	650	1,300	4,346
Revenue Equipment [Member] | Related Party [Member]
Related Party Transaction [Line Items]
Operating Leases, Rent Expense	0	813	3,223
Terminal Facilities [Member]
Related Party Transaction [Line Items]
Operating Leases, Rent Expense	3,900	2,200	3,900
Terminal Facilities [Member] | Related Party [Member]
Related Party Transaction [Line Items]
Operating Leases, Rent Expense	1,625	1,849	3,408
Related Party Transaction, Expenses from Transactions with Related Party	$ 1,600	$ 1,900	$ 3,600