Related Party Transactions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 12, 2011
Advisory Agreement
Related Party Transaction [Line Items]
Annual advisory fee					$ 0.1
Dedicated Carriers
Related Party Transaction [Line Items]
Related party payment	$ 6.6	$ 3.1	$ 13.2	$ 5.7
Facilities Lease
Related Party Transaction [Line Items]
Related party payment	0.3	0.8	0.7	1.5
Fuel Purchase Agreement
Related Party Transaction [Line Items]
Related party payment	0.6	0.3	1.2	0.8
Equipment Leases
Related Party Transaction [Line Items]
Related party payment	$ 0.8	$ 0.3	$ 1.5	0.6
Central Minnesota Logistics, Inc. [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	37.50%		37.50%
Central Minnesota Logistics, Inc. [Member] | Broker Commissions
Related Party Transaction [Line Items]
Related party payment	$ 0.7		$ 1.4	$ 1.3