Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 5,756.8	$ 5,059.7	$ 17,216.0	$ 14,001.8
Income before income taxes	668.8	581.8	2,057.8	1,569.0
Investment income	16.4	9.0	41.0	25.8
Depreciation and amortization	252.6	286.1	794.4	814.3
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	5,416.9	4,731.5	16,205.9	13,065.1
Income before income taxes	573.6	502.2	1,798.1	1,353.4
Depreciation and amortization	100.4	130.5	327.0	360.5
Truck, Parts and Other \| Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	4,429.4	3,867.4	13,250.2	10,551.1
Income before income taxes	391.0	346.7	1,220.2	890.6
Depreciation and amortization	93.6	122.4	306.9	340.5
Truck, Parts and Other \| Trucks \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	4,677.8	4,065.6	13,892.5	11,131.8
Truck, Parts and Other \| Trucks \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(248.4)	(198.2)	(642.3)	(580.7)
Truck, Parts and Other \| Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	960.1	840.0	2,868.0	2,449.8
Income before income taxes	188.5	151.8	574.8	453.8
Depreciation and amortization	2.3	2.4	6.7	6.3
Truck, Parts and Other \| Parts Subsegment \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	974.8	854.4	2,911.4	2,488.7
Truck, Parts and Other \| Parts Subsegment \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(14.7)	(14.4)	(43.4)	(38.9)
Truck, Parts and Other \| Other
Segment Reporting Information [Line Items]
Net sales and revenues	27.4	24.1	87.7	64.2
Income before income taxes	(5.9)	3.7	3.1	9.0
Depreciation and amortization	4.5	5.7	13.4	13.7
Financial Services
Segment Reporting Information [Line Items]
Revenues	339.9	328.2	1,010.1	936.7
Income before income taxes	78.8	70.6	218.7	189.8
Depreciation and amortization	$ 152.2	$ 155.6	$ 467.4	$ 453.8

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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