Statements of Consolidated Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Operating Revenue	$ 1,303.6	$ 1,251.2	$ 3,844.6	$ 3,682.4
Operating Expenses:
Salaries, wages and employee benefits	743.0	723.5	2,228.7	2,178.3
Fuel, operating expenses and supplies	233.6	216.6	705.8	642.6
Purchased transportation	183.4	169.1	516.0	463.2
Depreciation and amortization	34.9	36.7	110.2	111.0
Other operating expenses	65.6	60.6	188.8	187.4
Losses on property disposals, net	1.9	1.3	7.3	3.0
Total operating expenses	1,262.4	1,207.8	3,756.8	3,585.5
Operating Income	41.2	43.4	87.8	96.9
Nonoperating Expenses:
Interest expense	26.6	25.9	77.7	77.0
Non-union pension and postretirement benefits	6.9	3.3	6.0	9.8
Other, net	0.1	10.3	(0.8)	13.0
Nonoperating expenses, net	33.6	39.5	82.9	99.8
Income (loss) before income taxes	7.6	3.9	4.9	(2.9)
Income tax expense	4.7	0.9	2.2	0.4
Net income (loss)	2.9	3.0	2.7	(3.3)
Other comprehensive income, net of tax	12.2	2.5	18.5	12.9
Comprehensive Income	$ 15.1	$ 5.5	$ 21.2	$ 9.6
Average Common Shares Outstanding – Basic (in shares)	33,051	32,723	32,827	32,550
Average Common Shares Outstanding – Diluted (in shares)	33,995	33,592	33,755	32,550
Basic and Diluted Loss Per Share
Income (loss) Per Share – Basic (in dollars per share)	$ 0.09	$ 0.09	$ 0.08	$ (0.10)
Income (loss) Per Share – Diluted (in dollars per share)	$ 0.09	$ 0.09	$ 0.08	$ (0.10)