Compare Currently Reported Amounts and Reported Under LIFO in Consolidated Statements of Comprehensive Income (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		$ 4,659.4		$ 9,349.1
Income taxes	$ 201.7	144.8	$ 372.6	282.3
Net income	$ 720.4	$ 495.5	$ 1,320.9	$ 966.3
Net Income Per Share
Basic	$ 2.07	$ 1.42	$ 3.79	$ 2.78
Diluted	$ 2.07	$ 1.42	$ 3.79	$ 2.77
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ 5,811.0		$ 11,096.5
Income taxes	201.7		372.6
Net income	$ 720.4		$ 1,320.9
Net Income Per Share
Basic	$ 2.07		$ 3.79
Diluted	$ 2.07		$ 3.79
Pro Forma Under LIFO | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ 5,825.6		$ 11,122.4
Income taxes	198.1		366.2
Net income	$ 709.4		$ 1,301.4
Net Income Per Share
Basic	$ 2.04		$ 3.74
Diluted	$ 2.03		$ 3.73
Adjustment
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		$ (3.3)		$ (4.3)
Income taxes		0.7		1.0
Net income		$ 2.6		$ 3.3
Adjustment | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ (14.6)		$ (25.9)
Income taxes	3.6		6.4
Net income	$ 11.0		$ 19.5
Net Income Per Share
Basic	$ 0.03		$ 0.05
Diluted	$ 0.04		$ 0.06