Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 5,805.2	$ 4,704.2	$ 11,459.2	$ 8,942.1
Income before income taxes	719.6	539.0	1,389.0	987.2
Investment income	14.6	8.7	24.6	16.8
Depreciation and amortization	268.3	269.0	541.8	528.2
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	5,467.2	4,397.9	10,789.0	8,333.6
Income before income taxes	632.6	467.9	1,224.5	851.2
Depreciation and amortization	112.6	117.2	226.6	230.0
Truck, Parts and Other \| Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	4,467.8	3,553.6	8,820.8	6,683.7
Income before income taxes	434.0	312.7	829.2	543.9
Depreciation and amortization	106.0	111.0	213.3	218.1
Truck, Parts and Other \| Trucks \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	4,678.6	3,768.9	9,214.7	7,066.2
Truck, Parts and Other \| Trucks \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(210.8)	(215.3)	(393.9)	(382.5)
Truck, Parts and Other \| Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	968.0	823.1	1,907.9	1,609.8
Income before income taxes	194.5	151.2	386.3	302.0
Depreciation and amortization	2.2	2.0	4.4	3.9
Truck, Parts and Other \| Parts Subsegment \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	983.1	835.2	1,936.6	1,634.3
Truck, Parts and Other \| Parts Subsegment \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(15.1)	(12.1)	(28.7)	(24.5)
Truck, Parts and Other \| Other
Segment Reporting Information [Line Items]
Net sales and revenues	31.4	21.2	60.3	40.1
Income before income taxes	4.1	4.0	9.0	5.3
Depreciation and amortization	4.4	4.2	8.9	8.0
Financial Services
Segment Reporting Information [Line Items]
Revenues	338.0	306.3	670.2	608.5
Income before income taxes	72.4	62.4	139.9	119.2
Depreciation and amortization	$ 155.7	$ 151.8	$ 315.2	$ 298.2

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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