Statements of Consolidated Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Operating Revenue	$ 1,313.1	$ 1,015.4	$ 2,511.5	$ 2,165.8
Operating Expenses:
Salaries, wages and employee benefits	751.3	647.9	1,475.1	1,368.1
Fuel, operating expenses and supplies	217.0	162.7	420.5	370.7
Purchased transportation	210.3	126.0	410.3	262.2
Depreciation and amortization	35.0	34.2	68.3	69.9
Other operating expenses	72.2	55.2	136.6	116.8
(Gains) losses on property disposals, net	0.3	(6.0)	1.3	(45.3)
Total operating expenses	1,286.1	1,020.0	2,512.1	2,142.4
Operating Income (Loss)	27.0	(4.6)	(0.6)	23.4
Nonoperating Expenses:
Interest expense	37.7	40.2	73.6	68.5
Non-union pension and postretirement benefits	(1.1)	(1.6)	(2.4)	(3.2)
Other, net	(0.3)	1.4	(0.3)	(1.2)
Nonoperating expenses, net	36.3	40.0	70.9	64.1
Loss before income taxes	(9.3)	(44.6)	(71.5)	(40.7)
Income tax expense (benefit)	0.1	(7.5)	1.2	(7.9)
Net loss	(9.4)	(37.1)	(72.7)	(32.8)
Other comprehensive income, net of tax	3.3	3.2	6.9	4.5
Comprehensive Loss	$ (6.1)	$ (33.9)	$ (65.8)	$ (28.3)
Average Common Shares Outstanding - Basic	50,751	34,021	50,555	33,906
Average Common Shares Outstanding - Diluted	50,751	34,021	50,555	33,906
Loss Per Share - Basic	$ (0.18)	$ (1.09)	$ (1.44)	$ (0.97)
Loss Per Share - Diluted	$ (0.18)	$ (1.09)	$ (1.44)	$ (0.97)