Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 25,599.7	$ 23,495.7	$ 19,456.4
Income (loss) before income taxes	3,099.2	2,810.2	2,173.3
Investment income	82.3	60.9	35.3
Depreciation and amortization	1,077.3	1,054.1	1,107.5
Expenditures for long-lived assets	2,140.7	1,931.8	1,855.1
Assets	28,361.1	25,482.4	23,440.2
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	24,119.7	22,138.6	18,187.5
Income (loss) before income taxes	2,718.0	2,443.4	1,876.3
Depreciation and amortization	412.4	433.8	494.4
Expenditures for long-lived assets	762.1	846.7	847.1
Assets	12,289.7	11,082.8	10,237.9
Cash and marketable securities	5,169.4	4,299.6	3,621.9
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	19,989.5	18,187.0	14,774.8
Income (loss) before income taxes	1,904.9	1,672.1	1,253.8
Depreciation and amortization	381.9	406.2	468.2
Expenditures for long-lived assets	664.9	778.5	769.7
Assets	5,609.0	5,347.3	5,159.7
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	20,403.5	18,863.1	15,543.7
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(414.0)	(676.1)	(768.9)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	4,024.9	3,838.9	3,327.0
Income (loss) before income taxes	830.8	768.6	610.0
Depreciation and amortization	10.2	9.2	8.1
Expenditures for long-lived assets	46.2	29.4	23.4
Assets	1,172.1	1,090.9	950.7
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	4,073.1	3,896.2	3,380.2
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(48.2)	(57.3)	(53.2)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	105.3	112.7	85.7
Income (loss) before income taxes	(17.7)	2.7	12.5
Depreciation and amortization	20.3	18.4	18.1
Expenditures for long-lived assets	51.0	38.8	54.0
Assets	339.2	345.0	505.6
Financial Services
Segment Reporting Information [Line Items]
Revenues	1,480.0	1,357.1	1,268.9
Income (loss) before income taxes	298.9	305.9	261.7
Depreciation and amortization	664.9	620.3	613.1
Expenditures for long-lived assets	1,378.6	1,085.1	1,008.0
Assets	$ 16,071.4	$ 14,399.6	$ 13,202.3