Statements of Consolidated Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Operating Revenue	$ 1,326.5	$ 1,260.6	$ 2,541.0	$ 2,431.2
Operating Expenses:
Salaries, wages and employee benefits	756.0	736.4	1,485.7	1,454.8
Fuel, operating expenses and supplies	242.0	209.7	472.2	426.0
Purchased transportation	177.2	159.6	332.6	294.1
Depreciation and amortization	37.6	37.2	75.3	74.3
Other operating expenses	60.6	65.5	123.2	126.8
Losses (gains) on property disposals, net	2.2	(1.0)	5.4	1.7
Total operating expenses	1,275.6	1,207.4	2,494.4	2,377.7
Operating Income	50.9	53.2	46.6	53.5
Nonoperating Expenses:
Interest expense	25.5	25.7	51.1	51.1
Non-union pension and postretirement benefits	(0.4)	3.2	(0.9)	6.5
Other, net	1.0	1.7	(0.9)	2.7
Nonoperating expenses, net	26.1	30.6	49.3	60.3
Income (loss) before income taxes	24.8	22.6	(2.7)	(6.8)
Income tax expense (benefit)	10.4	3.6	(2.5)	(0.5)
Net income (loss)	14.4	19.0	(0.2)	(6.3)
Other comprehensive income, net of tax	4.3	6.0	6.3	10.4
Comprehensive Income	$ 18.7	$ 25.0	$ 6.1	$ 4.1
Average Common Shares Outstanding – Basic (in shares)	32,966	32,715	32,894	32,642
Average Common Shares Outstanding – Diluted (in shares)	33,794	33,322	32,894	32,642
Basic and Diluted Loss Per Share
Income (loss) Per Share – Basic (in dollars per share)	$ 0.44	$ 0.58	$ 0.00	$ (0.19)
Income (loss) Per Share – Diluted (in dollars per share)	$ 0.43	$ 0.57	$ 0.00	$ (0.19)