Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Revenues	$ 8,473.3	$ 6,472.6
Investment income	49.0	(2.5)
Total Income Before Income Taxes	919.0	771.4
Depreciation and amortization	221.1	185.7
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	8,050.1	6,106.4
Revenues	8,050.1	6,106.4
Income before income taxes	721.2	626.9
Depreciation and amortization	109.8	80.5
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	6,413.8	4,697.1
Income before income taxes	894.3	276.7
Depreciation and amortization	99.8	71.6
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	6,578.7	4,835.5
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(164.9)	(138.4)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,623.0	1,388.9
Income before income taxes	438.6	340.2
Depreciation and amortization	3.7	3.2
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,641.1	1,405.3
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(18.1)	(16.4)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	13.3	20.4
Income before income taxes	(611.7)	10.0
Depreciation and amortization	6.3	5.7
Financial Services
Segment Reporting Information [Line Items]
Revenues	423.2	366.2
Income before income taxes	148.8	147.0
Depreciation and amortization	$ 111.3	$ 105.2