Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Revenues	$ 7,158.7	$ 5,843.9	$ 13,631.3	$ 11,689.4
Investment income	5.4	5.0	2.9	9.9
Total Income Before Income Taxes	922.1	640.3	1,693.5	1,248.6
Depreciation and amortization	196.9	230.7	382.6	482.4
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	6,786.2	5,387.6	12,892.6	10,801.1
Revenues	6,786.2	5,387.6	12,892.6	10,801.1
Income before income taxes	772.3	528.8	1,399.2	1,055.8
Depreciation and amortization	87.2	77.1	167.7	157.2
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	5,336.4	4,152.2	10,033.5	8,385.2
Income before income taxes	422.1	256.5	698.8	526.5
Depreciation and amortization	77.7	68.8	149.3	140.7
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	5,499.5	4,295.2	10,335.0	8,694.4
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(163.1)	(143.0)	(301.5)	(309.2)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,434.7	1,211.3	2,823.6	2,372.0
Income before income taxes	353.3	266.8	693.5	518.4
Depreciation and amortization	3.3	3.0	6.5	5.8
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,450.6	1,226.4	2,855.9	2,402.6
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(15.9)	(15.1)	(32.3)	(30.6)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	15.1	24.1	35.5	43.9
Income before income taxes	(3.1)	5.5	6.9	10.9
Depreciation and amortization	6.2	5.3	11.9	10.7
Financial Services
Segment Reporting Information [Line Items]
Revenues	372.5	456.3	738.7	888.3
Income before income taxes	144.4	106.5	291.4	182.9
Depreciation and amortization	$ 109.7	$ 153.6	$ 214.9	$ 325.2