Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Revenues	$ 6,472.6	$ 5,845.5
Investment income	(2.5)	4.9
Total Income Before Income Taxes	771.4	608.3
Depreciation and amortization	185.7	251.7
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	6,106.4	5,413.5
Revenues	6,106.4	5,413.5
Income before income taxes	626.9	527.0
Depreciation and amortization	80.5	80.1
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	4,697.1	4,233.0
Income before income taxes	276.7	270.0
Depreciation and amortization	71.6	71.9
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	4,835.5	4,399.2
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(138.4)	(166.2)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,388.9	1,160.7
Income before income taxes	340.2	251.6
Depreciation and amortization	3.2	2.8
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,405.3	1,176.2
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(16.4)	(15.5)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	20.4	19.8
Income before income taxes	10.0	5.4
Depreciation and amortization	5.7	5.4
Financial Services
Segment Reporting Information [Line Items]
Revenues	366.2	432.0
Income before income taxes	147.0	76.4
Depreciation and amortization	$ 105.2	$ 171.6