Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Operating revenues	$ 1,444.5	$ 1,135.7	$ 4,033.9	$ 3,287.6
Operating expenses:
Purchased transportation	692.3	501.0	1,900.4	1,417.7
Salaries, wages, and benefits	290.2	259.2	832.9	771.4
Fuel and fuel taxes	70.3	49.5	204.1	152.5
Depreciation and amortization	74.2	74.2	220.5	216.3
Operating supplies and expenses	108.7	144.6	362.0	395.7
Insurance and related expenses	19.3	17.4	60.7	74.9
Other general expenses	35.8	26.0	97.6	78.0
Restructuring—net	0.0	0.5	0.0	(0.5)
Total operating expenses	1,290.8	1,072.4	3,678.2	3,106.0
Income from operations	153.7	63.3	355.7	181.6
Other expenses (income):
Interest income	(0.6)	(0.6)	(1.8)	(2.9)
Interest expense	3.3	3.4	9.7	10.5
Other expense (income)—net	4.0	0.4	(14.8)	(7.1)
Total other expenses (income)—net	6.7	3.2	(6.9)	0.5
Income before income taxes	147.0	60.1	362.6	181.1
Provision for income taxes	37.0	15.6	91.3	46.3
Net income	110.0	44.5	271.3	134.8
Other comprehensive income (loss):
Foreign currency translation gains (losses)—net	(0.1)	0.5	0.1	(0.2)
Net unrealized gains (losses) on marketable securities—net of tax	(0.2)	(0.1)	(0.6)	0.1
Total other comprehensive income (loss)—net	(0.3)	0.4	(0.5)	(0.1)
Comprehensive income	$ 109.7	$ 44.9	$ 270.8	$ 134.7
Weighted average common shares outstanding	177.7	177.3	177.5	177.2
Basic earnings per share	$ 0.62	$ 0.25	$ 1.53	$ 0.76
Weighted average diluted shares outstanding	177.9	177.7	177.8	177.5
Diluted earnings per share	$ 0.62	$ 0.25	$ 1.53	$ 0.76