Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Revenues	$ 8,239.9	$ 8,696.4	$ 25,756.3	$ 26,050.8
Investment income	108.7	80.8	290.0	192.5
Total Income Before Income Taxes	1,256.3	1,589.4	4,251.6	4,076.9
Depreciation and amortization	226.8	233.9	693.7	678.4
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net Sales and revenues	7,703.8	8,232.3	24,201.1	24,723.7
Revenues	7,703.8	8,232.3	24,201.1	24,723.7
Income (loss) before income taxes	1,041.1	1,374.8	3,630.0	3,457.1
Depreciation and amortization	99.0	112.1	313.2	330.7
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net Sales and revenues	6,027.0	6,636.4	19,145.8	19,877.7
Income (loss) before income taxes	630.8	960.9	2,349.7	2,803.5
Depreciation and amortization	90.7	102.1	283.7	301.1
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net Sales and revenues	6,110.8	6,772.8	19,464.0	20,274.5
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net Sales and revenues	(83.8)	(136.4)	(318.2)	(396.8)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net Sales and revenues	1,657.6	1,582.2	4,997.8	4,804.1
Income (loss) before income taxes	406.7	412.3	1,276.3	1,270.2
Depreciation and amortization	2.2	3.6	10.9	10.9
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net Sales and revenues	1,675.8	1,599.4	5,056.2	4,858.9
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net Sales and revenues	(18.2)	(17.2)	(58.4)	(54.8)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net Sales and revenues	19.2	13.7	57.5	41.9
Income (loss) before income taxes	3.6	1.6	4.0	(616.6)
Depreciation and amortization	6.1	6.4	18.6	18.7
Financial Services
Segment Reporting Information [Line Items]
Revenues	536.1	464.1	1,555.2	1,327.1
Income (loss) before income taxes	106.5	133.8	331.6	427.3
Depreciation and amortization	$ 127.8	$ 121.8	$ 380.5	$ 347.7