Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 8,696.4	$ 7,058.9	$ 26,050.8	$ 20,690.2
Investment income	80.8	21.4	192.5	24.3
Total Income Before Income Taxes	1,589.4	979.7	4,076.9	2,673.2
Depreciation and amortization	233.9	203.0	678.4	585.6
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	8,232.3	6,687.0	24,723.7	19,579.6
Revenues	8,232.3	6,687.0	24,723.7	19,579.6
Income before income taxes	1,374.8	812.1	3,457.1	2,211.3
Depreciation and amortization	112.1	95.8	330.7	263.5
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	6,636.4	5,198.2	19,877.7	15,231.7
Income before income taxes	960.9	430.5	2,803.5	1,129.3
Depreciation and amortization	102.1	86.3	301.1	235.6
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	6,772.8	5,389.8	20,274.5	15,724.8
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(136.4)	(191.6)	(396.8)	(493.1)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,582.2	1,471.5	4,804.1	4,295.1
Income before income taxes	412.3	373.6	1,270.2	1,067.1
Depreciation and amortization	3.6	3.5	10.9	10.0
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,599.4	1,487.3	4,858.9	4,343.2
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(17.2)	(15.8)	(54.8)	(48.1)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	13.7	17.3	41.9	52.8
Income before income taxes	1.6	8.0	(616.6)	14.9
Depreciation and amortization	6.4	6.0	18.7	17.9
Financial Services
Segment Reporting Information [Line Items]
Revenues	464.1	371.9	1,327.1	1,110.6
Income before income taxes	133.8	146.2	427.3	437.6
Depreciation and amortization	$ 121.8	$ 107.2	$ 347.7	$ 322.1