Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net sales and revenues										$ 19,456.4	$ 17,033.3	$ 19,115.1
Income before income taxes										2,173.3	1,130.4	2,337.1
Investment income										35.3	27.6	21.8
Depreciation and amortization										1,107.5	993.1	907.1
Expenditures for long-lived assets										1,855.1	1,992.4	1,747.0
Assets		$ 23,440.2				$ 20,638.9				23,440.2	20,638.9	21,109.8
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues		5,122.4	$ 4,731.5	$ 4,397.9	$ 3,935.7	3,767.0	$ 3,953.2	$ 4,115.8	$ 4,010.6	18,187.5	15,846.6	17,942.8
Income before income taxes										1,874.0	796.3	1,952.7
Depreciation and amortization										494.4	455.9	420.9
Expenditures for long-lived assets										847.1	778.0	702.6
Assets		10,237.9				8,444.1				10,237.9	8,444.1	8,855.2
Cash and marketable securities		3,621.9				2,922.6				3,621.9	2,922.6	3,378.0
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues										14,774.8	12,767.3	14,782.5
Income before income taxes										1,296.9	1,125.8	1,440.3
Depreciation and amortization										468.2	432.8	399.8
Expenditures for long-lived assets										769.7	735.6	660.0
Assets		5,159.7				4,429.4				5,159.7	4,429.4	4,472.3
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues										15,543.7	13,652.7	15,568.6
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues										(768.9)	(885.4)	(786.1)
Truck, Parts and Other | Parts
Segment Reporting Information [Line Items]
Net sales and revenues										3,327.0	3,005.7	3,060.1
Income before income taxes										614.2	543.8	555.6
Depreciation and amortization										8.1	7.3	6.2
Expenditures for long-lived assets										23.4	16.9	24.9
Assets		950.7				805.1				950.7	805.1	793.3
Truck, Parts and Other | Parts | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues										3,380.2	3,052.9	3,104.7
Truck, Parts and Other | Parts | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues										(53.2)	(47.2)	(44.6)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues										85.7	73.6	100.2
Income before income taxes	[1]									(37.1)	(873.3)	(43.2)
Depreciation and amortization										18.1	15.8	14.9
Expenditures for long-lived assets										54.0	25.5	17.7
Assets		505.6				287.0				505.6	287.0	211.6
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues		332.2	$ 328.2	$ 306.3	$ 302.2	303.7	$ 296.2	$ 297.4	$ 289.4	1,268.9	1,186.7	1,172.3
Income before income taxes										264.0	306.5	362.6
Depreciation and amortization										613.1	537.2	486.2
Expenditures for long-lived assets										1,008.0	1,214.4	1,044.4
Assets		$ 13,202.3				$ 12,194.8				$ 13,202.3	$ 12,194.8	$ 12,254.6

[1] Other includes the $833.0 European Commission charge in 2016.