Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Revenues	$ 7,441.7	$ 8,744.3
Cost of sales and revenues	5,891.0	6,673.8
Research and development	115.4	105.5
Selling, general and administrative	181.6	186.6
Interest and other borrowing expenses	190.6	160.0
Depreciation and other expenses	159.7	180.3
Provision for losses on receivables	18.3	16.1
Other segment (income) expenses	325.8	(27.0)
Income (Loss) before investment income and income taxes	559.3	1,449.0
Investment income	83.8	85.5
Total Income Before Income Taxes	643.1	1,534.5
Depreciation and amortization	205.1	235.0
Expenditures for long-lived assets	332.6	324.7
Assets	42,724.7		$ 43,418.9
Operating Segments
Segment Reporting Information [Line Items]
Revenues	7,625.5	8,901.6
Intersegment
Segment Reporting Information [Line Items]
Revenues	(183.8)	(157.3)
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	6,913.7	8,235.0
Revenues	6,913.7	8,235.0
Cost of sales and revenues	5,891.0	6,673.8
Research and development	115.4	105.5
Selling, general and administrative	143.3	147.6
Other segment (income) expenses	325.8	(27.0)
Income (Loss) before investment income and income taxes	438.2	1,335.1
Total Income Before Income Taxes	438.2	1,335.1
Depreciation and amortization	101.3	109.4
Expenditures for long-lived assets	180.1	166.4
Assets	19,979.5		21,007.4
Cash and marketable securities	8,100.0		9,649.9
Truck, Parts and Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	7,097.5	8,392.3
Truck, Parts and Other | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(183.8)	(157.3)
Truck, Parts and Other | Truck
Segment Reporting Information [Line Items]
Net sales and revenues	5,225.8	6,541.0
Cost of sales and revenues	4,716.5	5,521.2
Research and development	85.4	78.1
Selling, general and administrative	57.3	61.7
Other segment (income) expenses	1.7	(1.6)
Income (Loss) before investment income and income taxes	364.9	881.6
Total Income Before Income Taxes	364.9	881.6
Depreciation and amortization	90.9	99.6
Expenditures for long-lived assets	162.1	149.7
Assets	8,051.1		7,563.3
Truck, Parts and Other | Truck | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	5,390.8	6,678.9
Truck, Parts and Other | Truck | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(165.0)	(137.9)
Truck, Parts and Other | Parts
Segment Reporting Information [Line Items]
Net sales and revenues	1,689.9	1,675.9
Cost of sales and revenues	1,171.5	1,130.9
Research and development	30.0	27.2
Selling, general and administrative	60.9	61.3
Other segment (income) expenses	1.0	0.7
Income (Loss) before investment income and income taxes	426.5	455.8
Total Income Before Income Taxes	426.5	455.8
Depreciation and amortization	4.3	3.8
Expenditures for long-lived assets	3.3	5.5
Assets	2,207.9		2,097.0
Truck, Parts and Other | Parts | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,708.7	1,695.3
Truck, Parts and Other | Parts | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(18.8)	(19.4)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	(2.0)	18.1
Cost of sales and revenues	3.0	21.7
Research and development		0.2
Selling, general and administrative	25.1	24.6
Other segment (income) expenses	323.1	(26.1)
Income (Loss) before investment income and income taxes	(353.2)	(2.3)
Total Income Before Income Taxes	(353.2)	(2.3)
Depreciation and amortization	6.1	6.0
Expenditures for long-lived assets	14.7	11.2
Assets	1,620.5		1,697.2
Truck, Parts and Other | Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	(2.0)	18.1
Financial Services
Segment Reporting Information [Line Items]
Revenues	528.0	509.3
Selling, general and administrative	38.3	39.0
Interest and other borrowing expenses	190.6	160.0
Depreciation and other expenses	159.7	180.3
Provision for losses on receivables	18.3	16.1
Income (Loss) before investment income and income taxes	121.1	113.9
Total Income Before Income Taxes	121.1	113.9
Depreciation and amortization	103.8	125.6
Expenditures for long-lived assets	152.5	158.3
Assets	22,745.2		$ 22,411.5
Financial Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	$ 528.0	$ 509.3