Statements of Consolidated Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Operating Revenue	$ 1,251.2	$ 1,221.3	$ 3,682.4	$ 3,549.2
Operating Expenses:
Salaries, wages and employee benefits	726.8	715.8	2,188.1	2,132.6
Operating expenses and supplies	216.6	206.9	642.6	595.7
Purchased transportation	169.1	156.8	463.2	409.0
Depreciation and amortization	36.7	40.3	111.0	119.5
Other operating expenses	60.6	62.5	187.4	194.2
(Gains) losses on property disposals, net	1.3	0.2	3.0	(11.2)
Total operating expenses	1,211.1	1,182.5	3,595.3	3,439.8
Operating Income	40.1	38.8	87.1	109.4
Nonoperating Expenses:
Interest expense	25.9	25.6	77.0	77.9
Other, net	10.3	(1.2)	13.0	(0.9)
Nonoperating expenses, net	36.2	24.4	90.0	77.0
Income (loss) before income taxes	3.9	14.4	(2.9)	32.4
Income tax expense	0.9	0.5	0.4	3.4
Net income (loss)	3.0	13.9	(3.3)	29.0
Other comprehensive income, net of tax	2.5	2.3	12.9	2.9
Comprehensive Income	$ 5.5	$ 16.2	$ 9.6	$ 31.9
Average Common Shares Outstanding – Basic (in shares)	32,723	32,466	32,550	32,398
Average Common Shares Outstanding – Diluted (in shares)	33,592	33,194	32,550	32,915
Basic and Diluted Loss Per Share
Income (loss) Per Share – Basic (in dollars per share)	$ 0.09	$ 0.43	$ (0.10)	$ 0.89
Income (loss) Per Share – Diluted (in dollars per share)	$ 0.09	$ 0.42	$ (0.10)	$ 0.88