Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 6,367.0	$ 5,756.8	$ 19,482.5	$ 17,216.0
Income before income taxes	777.6	668.8	2,411.4	2,057.8
Investment income	21.1	16.4	62.2	41.0
Depreciation and amortization	272.1	252.6	792.4	794.4
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	6,004.2	5,416.9	18,408.8	16,205.9
Income before income taxes	690.0	573.6	2,118.4	1,798.1
Depreciation and amortization	102.4	100.4	311.0	327.0
Truck, Parts and Other \| Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	4,977.4	4,429.4	15,296.6	13,250.2
Income before income taxes	481.5	391.0	1,509.2	1,220.2
Depreciation and amortization	95.8	93.6	290.5	306.9
Truck, Parts and Other \| Trucks \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	5,102.6	4,677.8	15,638.5	13,892.5
Truck, Parts and Other \| Trucks \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(125.2)	(248.4)	(341.9)	(642.3)
Truck, Parts and Other \| Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,000.9	960.1	3,031.0	2,868.0
Income before income taxes	207.4	188.5	625.6	574.8
Depreciation and amortization	2.5	2.3	7.8	6.7
Truck, Parts and Other \| Parts Subsegment \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,012.8	974.8	3,066.6	2,911.4
Truck, Parts and Other \| Parts Subsegment \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(11.9)	(14.7)	(35.6)	(43.4)
Truck, Parts and Other \| Other
Segment Reporting Information [Line Items]
Net sales and revenues	25.9	27.4	81.2	87.7
Income before income taxes	1.1	(5.9)	(16.4)	3.1
Depreciation and amortization	4.1	4.5	12.7	13.4
Financial Services
Segment Reporting Information [Line Items]
Revenues	362.8	339.9	1,073.7	1,010.1
Income before income taxes	66.5	78.8	230.8	218.7
Depreciation and amortization	$ 169.7	$ 152.2	$ 481.4	$ 467.4

The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production.

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