Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenues	$ 7,058.9	$ 5,146.8	$ 20,690.2	$ 16,836.2
Investment income	21.4	1.6	24.3	11.5
Total Income Before Income Taxes	979.7	487.2	2,673.2	1,735.8
Depreciation and amortization	203.0	209.9	585.6	692.3
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	6,687.0	4,737.7	19,579.6	15,538.8
Revenues	6,687.0	4,737.7	19,579.6	15,538.8
Income before income taxes	812.1	365.5	2,211.3	1,421.3
Depreciation and amortization	95.8	71.6	263.5	228.8
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	5,198.2	3,452.6	15,231.7	11,837.8
Income before income taxes	430.5	80.4	1,129.3	606.9
Depreciation and amortization	86.3	63.5	235.6	204.2
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	5,389.8	3,583.2	15,724.8	12,277.6
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(191.6)	(130.6)	(493.1)	(439.8)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,471.5	1,260.2	4,295.1	3,632.2
Income before income taxes	373.6	282.1	1,067.1	800.5
Depreciation and amortization	3.5	2.8	10.0	8.6
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,487.3	1,275.2	4,343.2	3,677.8
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(15.8)	(15.0)	(48.1)	(45.6)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	17.3	24.9	52.8	68.8
Income before income taxes	8.0	3.0	14.9	13.9
Depreciation and amortization	6.0	5.3	17.9	16.0
Financial Services
Segment Reporting Information [Line Items]
Revenues	371.9	409.1	1,110.6	1,297.4
Income before income taxes	146.2	120.1	437.6	303.0
Depreciation and amortization	$ 107.2	$ 138.3	$ 322.1	$ 463.5