Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 6,627.9	$ 5,805.2	$ 13,115.5	$ 11,459.2
Income before income taxes	814.4	719.6	1,633.8	1,389.0
Investment income	21.8	14.6	41.1	24.6
Depreciation and amortization	261.2	268.3	520.3	541.8
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	6,266.5	5,467.2	12,404.6	10,789.0
Income before income taxes	712.3	632.6	1,428.4	1,224.5
Depreciation and amortization	104.5	112.6	208.6	226.6
Truck, Parts and Other \| Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	5,211.9	4,467.8	10,319.2	8,820.8
Income before income taxes	510.7	434.0	1,027.7	829.2
Depreciation and amortization	97.3	106.0	194.7	213.3
Truck, Parts and Other \| Trucks \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	5,309.3	4,678.6	10,535.9	9,214.7
Truck, Parts and Other \| Trucks \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(97.4)	(210.8)	(216.7)	(393.9)
Truck, Parts and Other \| Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,025.4	968.0	2,030.1	1,907.9
Income before income taxes	210.6	194.5	418.2	386.3
Depreciation and amortization	2.6	2.2	5.3	4.4
Truck, Parts and Other \| Parts Subsegment \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,037.8	983.1	2,053.8	1,936.6
Truck, Parts and Other \| Parts Subsegment \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(12.4)	(15.1)	(23.7)	(28.7)
Truck, Parts and Other \| Other
Segment Reporting Information [Line Items]
Net sales and revenues	29.2	31.4	55.3	60.3
Income before income taxes	(9.0)	4.1	(17.5)	9.0
Depreciation and amortization	4.6	4.4	8.6	8.9
Financial Services
Segment Reporting Information [Line Items]
Revenues	361.4	338.0	710.9	670.2
Income before income taxes	80.3	72.4	164.3	139.9
Depreciation and amortization	$ 156.7	$ 155.7	$ 311.7	$ 315.2

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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