Segment Reporting - Additional Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment									3
Operating revenues | $	$ 1,191.2	$ 1,110.8	$ 1,075.2	$ 1,006.4	$ 1,069.9	$ 1,053.2	$ 994.6	$ 928.1	$ 4,383.6	$ 4,045.7	$ 3,959.4
Other
Segment Reporting Information [Line Items]
Operating revenues | $									293.6	240.5	255.5
Other | Other Insurance
Segment Reporting Information [Line Items]
Operating revenues | $									$ 78.4	$ 54.4	$ 43.4
Truckload
Segment Reporting Information [Line Items]
Number of operating segments | Segment									3
Logistics
Segment Reporting Information [Line Items]
Number of operating segments | Segment									3