Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 6,487.6	$ 5,654.0
Income before income taxes	819.4	669.4
Investment income	19.3	10.0
Depreciation and amortization	259.1	273.5
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	6,138.1	5,321.8
Income before income taxes	716.1	591.9
Depreciation and amortization	104.1	114.0
Truck, Parts and Other \| Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	5,107.3	4,353.0
Income before income taxes	517.0	395.2
Depreciation and amortization	97.4	107.3
Truck, Parts and Other \| Trucks \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	5,226.6	4,536.1
Truck, Parts and Other \| Trucks \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(119.3)	(183.1)
Truck, Parts and Other \| Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,004.7	939.9
Income before income taxes	207.6	191.8
Depreciation and amortization	2.7	2.2
Truck, Parts and Other \| Parts Subsegment \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,016.0	953.5
Truck, Parts and Other \| Parts Subsegment \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(11.3)	(13.6)
Truck, Parts and Other \| Other
Segment Reporting Information [Line Items]
Net sales and revenues	26.1	28.9
Income before income taxes	(8.5)	4.9
Depreciation and amortization	4.0	4.5
Financial Services
Segment Reporting Information [Line Items]
Revenues	349.5	332.2
Income before income taxes	84.0	67.5
Depreciation and amortization	$ 155.0	$ 159.5

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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