Compare Currently Reported Amounts and Reported Under LIFO in Consolidated Statements of Comprehensive Income (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		$ 4,689.7
Income taxes	$ 170.9	137.5
Net income	$ 600.5	$ 470.8
Net Income Per Share
Basic	$ 1.72	$ 1.35
Diluted	$ 1.72	$ 1.35
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ 5,285.5
Income taxes	170.9
Net income	$ 600.5
Net Income Per Share
Basic	$ 1.72
Diluted	$ 1.72
Pro Forma Under LIFO | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ 5,296.8
Income taxes	168.1
Net income	$ 592.0
Net Income Per Share
Basic	$ 1.70
Diluted	$ 1.70
Adjustment
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		$ (1.0)
Income taxes		0.3
Net income		$ 0.7
Adjustment | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ (11.3)
Income taxes	2.8
Net income	$ 8.5
Net Income Per Share
Basic	$ 0.02
Diluted	$ 0.02