Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 18,728.5	$ 25,599.7	$ 23,495.7
Income (loss) before income taxes	1,657.9	3,099.2	2,810.2
Investment income	35.9	82.3	60.9
Depreciation and amortization	1,049.0	1,077.3	1,054.1
Expenditures for long-lived assets	1,657.5	2,140.7	1,931.8
Assets	28,260.0	28,361.1	25,482.4
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	17,154.3	24,119.7	22,138.6
Income (loss) before income taxes	1,398.9	2,718.0	2,443.4
Depreciation and amortization	323.8	412.4	433.8
Expenditures for long-lived assets	610.8	762.1	846.7
Assets	12,460.2	12,289.7	11,082.8
Cash and marketable securities	4,834.0	5,169.4	4,299.6
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	13,164.8	19,989.5	18,187.0
Income (loss) before income taxes	581.4	1,904.9	1,672.1
Depreciation and amortization	292.2	381.9	406.2
Expenditures for long-lived assets	556.0	664.9	778.5
Assets	5,971.9	5,609.0	5,347.3
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	13,643.0	20,403.5	18,863.1
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(478.2)	(414.0)	(676.1)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	3,912.9	4,024.9	3,838.9
Income (loss) before income taxes	799.3	830.8	768.6
Depreciation and amortization	10.8	10.2	9.2
Expenditures for long-lived assets	18.4	46.2	29.4
Assets	1,255.6	1,172.1	1,090.9
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	3,962.7	4,073.1	3,896.2
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(49.8)	(48.2)	(57.3)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	76.6	105.3	112.7
Income (loss) before income taxes	18.2	(17.7)	2.7
Depreciation and amortization	20.8	20.3	18.4
Expenditures for long-lived assets	36.4	51.0	38.8
Assets	398.7	339.2	345.0
Financial Services
Segment Reporting Information [Line Items]
Revenues	1,574.2	1,480.0	1,357.1
Income (loss) before income taxes	223.1	298.9	305.9
Depreciation and amortization	725.2	664.9	620.3
Expenditures for long-lived assets	1,046.7	1,378.6	1,085.1
Assets	$ 15,799.8	$ 16,071.4	$ 14,399.6