Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenues	$ 5,845.5	$ 5,161.7
Income before income taxes	607.3	462.2
Investment income	4.9	14.8
Depreciation and amortization	251.7	272.0
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	5,413.5	4,778.0
Income before income taxes	526.0	399.1
Depreciation and amortization	80.1	90.8
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	4,233.0	3,757.6
Income before income taxes	269.5	183.1
Depreciation and amortization	71.9	83.2
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	4,399.2	3,863.2
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(166.2)	(105.6)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,160.7	998.6
Income before income taxes	251.3	214.7
Depreciation and amortization	2.8	2.5
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,176.2	1,010.7
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(15.5)	(12.1)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	19.8	21.8
Income before income taxes	5.2	1.3
Depreciation and amortization	5.4	5.1
Financial Services
Segment Reporting Information [Line Items]
Revenues	432.0	383.7
Income before income taxes	76.4	48.3
Depreciation and amortization	$ 171.6	$ 181.2