Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Revenues	$ 6,671.8	$ 8,239.9	$ 21,624.0	$ 25,756.3
Cost of sales and revenues	5,345.4	6,427.7	17,231.8	19,873.8
Research and development	111.0	115.0	339.3	337.6
Selling, general and administrative	180.2	186.5	541.1	556.6
Interest and other borrowing expenses	201.0	188.4	591.9	520.9
Depreciation and other expenses	161.7	176.6	476.3	530.5
Provision for losses on receivables	36.5	22.4	84.0	50.2
Other segment expenses (income)	(16.8)	(24.3)	299.5	(74.9)
Income (Loss) before investment income and income taxes	652.8	1,147.6	2,060.1	3,961.6
Investment income	90.8	108.7	258.5	290.0
Total Income Before Income Taxes	743.6	1,256.3	2,318.6	4,251.6
Depreciation and amortization	205.8	226.8	618.2	693.7
Expenditures for long-lived assets	295.7	368.2	1,016.2	1,205.1
Assets	44,201.2		44,201.2		$ 43,418.9
Operating Segments
Segment Reporting Information [Line Items]
Revenues	6,763.6	8,341.9	22,015.6	26,132.9
Intersegment
Segment Reporting Information [Line Items]
Revenues	(91.8)	(102.0)	(391.6)	(376.6)
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	6,106.5	7,703.8	19,983.0	24,201.1
Revenues	6,106.5	7,703.8	19,983.0	24,201.1
Cost of sales and revenues	5,345.4	6,427.7	17,231.8	19,873.8
Research and development	111.0	115.0	339.3	337.6
Selling, general and administrative	140.3	144.3	422.8	434.6
Other segment expenses (income)	(16.8)	(24.3)	299.5	(74.9)
Income (Loss) before investment income and income taxes	526.6	1,041.1	1,689.6	3,630.0
Total Income Before Income Taxes	526.6	1,041.1	1,689.6	3,630.0
Depreciation and amortization	99.9	99.0	303.6	313.2
Expenditures for long-lived assets	152.7	180.7	554.6	577.6
Assets	21,180.7		21,180.7		21,007.4
Cash and marketable securities	9,068.6		9,068.6		9,649.9
Truck, Parts and Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	6,198.3	7,805.8	20,374.6	24,577.7
Truck, Parts and Other | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(91.8)	(102.0)	(391.6)	(376.6)
Truck, Parts and Other | Truck
Segment Reporting Information [Line Items]
Net sales and revenues	4,381.4	6,027.0	14,850.3	19,145.8
Cost of sales and revenues	4,125.8	5,246.0	13,630.1	16,356.6
Research and development	79.6	87.5	246.9	255.4
Selling, general and administrative	63.3	61.9	175.3	183.3
Other segment expenses (income)	10.2	0.8	21.8	0.8
Income (Loss) before investment income and income taxes	102.5	630.8	776.2	2,349.7
Total Income Before Income Taxes	102.5	630.8	776.2	2,349.7
Depreciation and amortization	89.6	90.7	273.6	283.7
Expenditures for long-lived assets	137.1	156.7	512.6	508.0
Assets	8,096.1		8,096.1		7,563.3
Truck, Parts and Other | Truck | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	4,455.9	6,110.8	15,187.6	19,464.0
Truck, Parts and Other | Truck | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(74.5)	(83.8)	(337.3)	(318.2)
Truck, Parts and Other | Parts
Segment Reporting Information [Line Items]
Net sales and revenues	1,724.6	1,657.6	5,135.4	4,997.8
Cost of sales and revenues	1,216.5	1,159.2	3,592.7	3,450.7
Research and development	31.4	27.4	92.4	81.7
Selling, general and administrative	66.5	62.5	192.0	186.4
Other segment expenses (income)	0.2	1.8	5.3	2.7
Income (Loss) before investment income and income taxes	410.0	406.7	1,253.0	1,276.3
Total Income Before Income Taxes	410.0	406.7	1,253.0	1,276.3
Depreciation and amortization	4.3	2.2	11.5	10.9
Expenditures for long-lived assets	8.3	14.7	14.7	36.7
Assets	2,312.7		2,312.7		2,097.0
Truck, Parts and Other | Parts | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,741.9	1,675.8	5,189.7	5,056.2
Truck, Parts and Other | Parts | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(17.3)	(18.2)	(54.3)	(58.4)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	0.5	19.2	(2.7)	57.5
Cost of sales and revenues	3.1	22.5	9.0	66.5
Research and development		0.1		0.5
Selling, general and administrative	10.5	19.9	55.5	64.9
Other segment expenses (income)	(27.2)	(26.9)	272.4	(78.4)
Income (Loss) before investment income and income taxes	14.1	3.6	(339.6)	4.0
Total Income Before Income Taxes	14.1	3.6	(339.6)	4.0
Depreciation and amortization	6.0	6.1	18.5	18.6
Expenditures for long-lived assets	7.3	9.3	27.3	32.9
Assets	1,703.3		1,703.3		1,697.2
Truck, Parts and Other | Other | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	0.5	19.2	(2.7)	57.5
Financial Services
Segment Reporting Information [Line Items]
Revenues	565.3	536.1	1,641.0	1,555.2
Selling, general and administrative	39.9	42.2	118.3	122.0
Interest and other borrowing expenses	201.0	188.4	591.9	520.9
Depreciation and other expenses	161.7	176.6	476.3	530.5
Provision for losses on receivables	36.5	22.4	84.0	50.2
Income (Loss) before investment income and income taxes	126.2	106.5	370.5	331.6
Total Income Before Income Taxes	126.2	106.5	370.5	331.6
Depreciation and amortization	105.9	127.8	314.6	380.5
Expenditures for long-lived assets	143.0	187.5	461.6	627.5
Assets	23,020.5		23,020.5		$ 22,411.5
Financial Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	$ 565.3	$ 536.1	$ 1,641.0	$ 1,555.2