Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 4,936.0	$ 6,367.0	$ 13,159.9	$ 19,482.5
Income (loss) before income taxes	489.9	777.6	1,136.1	2,411.4
Investment income	6.4	21.1	30.2	62.2
Depreciation and amortization	266.2	272.1	779.5	792.4
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	4,538.4	6,004.2	12,018.3	18,408.8
Income (loss) before income taxes	428.0	690.0	946.6	2,118.4
Depreciation and amortization	81.8	102.4	242.6	311.0
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	3,504.0	4,977.4	9,120.0	15,296.6
Income (loss) before income taxes	210.1	481.5	347.0	1,509.2
Depreciation and amortization	73.3	95.8	218.8	290.5
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	3,675.8	5,102.6	9,469.9	15,638.5
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(171.8)	(125.2)	(349.9)	(341.9)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,016.2	1,000.9	2,838.5	3,031.0
Income (loss) before income taxes	210.2	207.4	576.8	625.6
Depreciation and amortization	3.0	2.5	8.0	7.8
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,029.4	1,012.8	2,874.7	3,066.6
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(13.2)	(11.9)	(36.2)	(35.6)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	18.2	25.9	59.8	81.2
Income (loss) before income taxes	7.7	1.1	22.8	(16.4)
Depreciation and amortization	5.5	4.1	15.8	12.7
Financial Services
Segment Reporting Information [Line Items]
Revenues	397.6	362.8	1,141.6	1,073.7
Income (loss) before income taxes	55.5	66.5	159.3	230.8
Depreciation and amortization	$ 184.4	$ 169.7	$ 536.9	$ 481.4