Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 8,881.1	$ 7,158.7	$ 17,354.4	$ 13,631.3
Investment income	62.7	5.4	111.7	2.9
Total Income Before Income Taxes	1,568.5	922.1	2,487.5	1,693.5
Depreciation and amortization	223.4	196.9	444.5	382.6
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	8,441.3	6,786.2	16,491.4	12,892.6
Revenues	8,441.3	6,786.2	16,491.4	12,892.6
Income before income taxes	1,361.1	772.3	2,082.3	1,399.2
Depreciation and amortization	108.8	87.2	218.6	167.7
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	6,827.5	5,336.4	13,241.3	10,033.5
Income before income taxes	948.3	422.1	1,842.6	698.8
Depreciation and amortization	99.2	77.7	199.0	149.3
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	6,923.0	5,499.5	13,501.7	10,335.0
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(95.5)	(163.1)	(260.4)	(301.5)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,598.9	1,434.7	3,221.9	2,823.6
Income before income taxes	419.3	353.3	857.9	693.5
Depreciation and amortization	3.6	3.3	7.3	6.5
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,618.4	1,450.6	3,259.5	2,855.9
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(19.5)	(15.9)	(37.6)	(32.3)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	14.9	15.1	28.2	35.5
Income before income taxes	(6.5)	(3.1)	(618.2)	6.9
Depreciation and amortization	6.0	6.2	12.3	11.9
Financial Services
Segment Reporting Information [Line Items]
Revenues	439.8	372.5	863.0	738.7
Income before income taxes	144.7	144.4	293.5	291.4
Depreciation and amortization	$ 114.6	$ 109.7	$ 225.9	$ 214.9