Statements of Consolidated Comprehensive Income (Loss) (Unaudited) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Operating Revenue	$ 1,301.4	$ 1,183.4	$ 3,812.9	$ 3,349.2
Operating Expenses:
Salaries, wages and employee benefits	729.7	720.6	2,204.8	2,088.7
Fuel, operating expenses and supplies	216.1	175.4	636.6	546.1
Purchased transportation	200.3	177.1	610.6	439.3
Depreciation and amortization	37.8	32.5	106.1	102.4
Other operating expenses	68.9	58.4	205.5	175.2
(Gains) losses on property disposals, net	0.2		1.5	(45.3)
Total operating expenses	1,253.0	1,164.0	3,765.1	3,306.4
Operating Income	48.4	19.4	47.8	42.8
Nonoperating Expenses:
Interest expense	38.6	33.4	112.2	101.9
Non-union pension and postretirement expense (benefits)	1.7	(1.1)	(0.7)	(4.3)
Other, net	(0.2)		(0.5)	(1.2)
Nonoperating expenses, net	40.1	32.3	111.0	96.4
Income (loss) before income taxes	8.3	(12.9)	(63.2)	(53.6)
Income tax expense (benefit)	0.0	(10.9)	1.2	(18.8)
Net income (loss)	8.3	(2.0)	(64.4)	(34.8)
Other comprehensive income (loss), net of tax	(28.8)	98.0	(21.9)	102.5
Comprehensive Income (Loss)	$ (20.5)	$ 96.0	$ (86.3)	$ 67.7
Average Common Shares Outstanding - Basic	50,868	48,672	50,661	38,864
Average Common Shares Outstanding - Diluted	51,818	48,672	50,661	38,864
Earnings (Loss) Per Share - Basic	$ 0.16	$ (0.04)	$ (1.27)	$ (0.90)
Earnings (Loss) Per Share - Diluted	$ 0.16	$ (0.04)	$ (1.27)	$ (0.90)