Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Net sales and revenues	$ 5,654.0	$ 4,237.9
Income before income taxes	669.4	448.2
Investment income	10.0	8.1
Depreciation and amortization	273.5	259.2
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	5,321.8	3,935.7
Income before income taxes	591.9	383.3
Depreciation and amortization	114.0	112.8
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	4,353.0	3,130.1
Income before income taxes	395.2	231.2
Depreciation and amortization	107.3	107.1
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	4,536.1	3,297.3
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(183.1)	(167.2)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	939.9	786.7
Income before income taxes	191.8	150.8
Depreciation and amortization	2.2	1.9
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	953.5	799.1
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(13.6)	(12.4)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	28.9	18.9
Income before income taxes	4.9	1.3
Depreciation and amortization	4.5	3.8
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	332.2	302.2
Income before income taxes	67.5	56.8
Depreciation and amortization	$ 159.5	$ 146.4