Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Revenues	$ 8,772.1	$ 8,881.1	$ 17,516.4	$ 17,354.4
Investment income	95.8	62.7	181.3	111.7
Total Income Before Income Taxes	1,460.8	1,568.5	2,995.3	2,487.5
Depreciation and amortization	231.9	223.4	466.9	444.5
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	8,262.3	8,441.3	16,497.3	16,491.4
Revenues	8,262.3	8,441.3	16,497.3	16,491.4
Income (loss) before income taxes	1,253.8	1,361.1	2,588.9	2,082.3
Depreciation and amortization	104.8	108.8	214.2	218.6
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	6,577.8	6,827.5	13,118.8	13,241.3
Income (loss) before income taxes	837.3	948.3	1,718.9	1,842.6
Depreciation and amortization	93.4	99.2	193.0	199.0
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	6,674.3	6,923.0	13,353.2	13,501.7
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(96.5)	(95.5)	(234.4)	(260.4)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,664.3	1,598.9	3,340.2	3,221.9
Income (loss) before income taxes	413.8	419.3	869.6	857.9
Depreciation and amortization	4.9	3.6	8.7	7.3
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,685.1	1,618.4	3,380.4	3,259.5
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(20.8)	(19.5)	(40.2)	(37.6)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	20.2	14.9	38.3	28.2
Income (loss) before income taxes	2.7	(6.5)	0.4	(618.2)
Depreciation and amortization	6.5	6.0	12.5	12.3
Financial Services
Segment Reporting Information [Line Items]
Revenues	509.8	439.8	1,019.1	863.0
Income (loss) before income taxes	111.2	144.7	225.1	293.5
Depreciation and amortization	$ 127.1	$ 114.6	$ 252.7	$ 225.9