Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 5,146.8	$ 4,936.0	$ 16,836.2	$ 13,159.9
Income before income taxes	483.3	489.9	1,727.6	1,136.1
Investment income	1.6	6.4	11.5	30.2
Depreciation and amortization	209.9	266.2	692.3	779.5
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	4,737.7	4,538.4	15,538.8	12,018.3
Income before income taxes	361.6	428.0	1,413.1	946.6
Depreciation and amortization	71.6	81.8	228.8	242.6
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	3,452.6	3,504.0	11,837.8	9,120.0
Income before income taxes	78.5	210.1	603.0	347.0
Depreciation and amortization	63.5	73.3	204.2	218.8
Truck, Parts and Other | Trucks | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	3,555.1	3,675.8	12,249.5	9,469.9
Truck, Parts and Other | Trucks | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(102.5)	(171.8)	(411.7)	(349.9)
Truck, Parts and Other | Parts Subsegment
Segment Reporting Information [Line Items]
Net sales and revenues	1,260.2	1,016.2	3,632.2	2,838.5
Income before income taxes	280.8	210.2	797.7	576.8
Depreciation and amortization	2.8	3.0	8.6	8.0
Truck, Parts and Other | Parts Subsegment | Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	1,275.2	1,029.4	3,677.8	2,874.7
Truck, Parts and Other | Parts Subsegment | Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(15.0)	(13.2)	(45.6)	(36.2)
Truck, Parts and Other | Other
Segment Reporting Information [Line Items]
Net sales and revenues	24.9	18.2	68.8	59.8
Income before income taxes	2.3	7.7	12.4	22.8
Depreciation and amortization	5.3	5.5	16.0	15.8
Financial Services
Segment Reporting Information [Line Items]
Revenues	409.1	397.6	1,297.4	1,141.6
Income before income taxes	120.1	55.5	303.0	159.3
Depreciation and amortization	$ 138.3	$ 184.4	$ 463.5	$ 536.9