Statements of Consolidated Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Operating Revenue	$ 1,260.6	$ 1,207.6	$ 2,431.2	$ 2,327.9
Operating Expenses:
Salaries, wages and employee benefits	739.6	718.7	1,461.3	1,416.8
Operating expenses and supplies	209.7	198.6	426.0	388.8
Purchased transportation	159.6	136.7	294.1	252.2
Depreciation and amortization	37.2	38.5	74.3	79.2
Other operating expenses	65.5	69.0	126.8	131.7
(Gains) losses on property disposals, net	(1.0)	(11.1)	1.7	(11.4)
Total operating expenses	1,210.6	1,150.4	2,384.2	2,257.3
Operating Income	50.0	57.2	47.0	70.6
Nonoperating Expenses:
Interest expense	25.7	26.2	51.1	52.3
Other, net	1.7	(0.8)	2.7	0.3
Nonoperating expenses, net	27.4	25.4	53.8	52.6
Income (loss) before income taxes	22.6	31.8	(6.8)	18.0
Income tax expense (benefit)	3.6	4.7	(0.5)	2.9
Net income (loss)	19.0	27.1	(6.3)	15.1
Other comprehensive income, net of tax	6.0	3.2	10.4	0.6
Comprehensive Income	$ 25.0	$ 30.3	$ 4.1	$ 15.7
Average Common Shares Outstanding – Basic (in shares)	32,715	32,459	32,642	32,362
Average Common Shares Outstanding – Diluted (in shares)	33,322	32,854	32,642	32,814
Basic and Diluted Loss Per Share
Earnings (Loss) Per Share – Basic (in dollars per share)	$ 0.58	$ 0.84	$ (0.19)	$ 0.47
Loss Per Share – Diluted (in dollars per share)	$ 0.57	$ 0.83	$ (0.19)	$ 0.46