Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Net sales and revenues	$ 4,928.4	$ 4,300.1	$ 13,877.9	$ 12,524.9
Investment income	5.7	7.3	17.0	21.8
Income (loss) before income taxes	552.4	440.0	1,439.9	1,210.1
Depreciation and amortization	230.1	206.4	677.6	588.7
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	4,622.5	4,006.6	12,975.7	11,649.5
Income (loss) before income taxes	449.8	344.5	1,148.8	938.5
Depreciation and amortization	108.1	92.9	316.3	262.2
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	305.9	293.5	902.2	875.4
Income (loss) before income taxes	96.9	88.2	274.1	249.8
Depreciation and amortization	122.0	113.5	361.3	326.5
Trucks
Segment Reporting Information [Line Items]
Revenue	4,000.1	3,416.6	11,168.1	9,924.6
Income (loss) before income taxes	330.0	242.5	801.9	645.9
Depreciation and amortization	103.5	89.2	303.7	250.9
Trucks | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	3,810.6	3,261.2	10,594.8	9,464.5
Trucks | Intersegment
Segment Reporting Information [Line Items]
Revenue	(189.5)	(155.4)	(573.3)	(460.1)
Parts
Segment Reporting Information [Line Items]
Revenue	797.0	726.2	2,324.3	2,126.0
Income (loss) before income taxes	127.9	106.5	366.7	311.2
Depreciation and amortization	1.5	1.2	4.2	3.9
Parts | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	784.2	715.3	2,288.8	2,092.2
Parts | Intersegment
Segment Reporting Information [Line Items]
Revenue	(12.8)	(10.9)	(35.5)	(33.8)
Other products
Segment Reporting Information [Line Items]
Net sales and revenues	27.7	30.1	92.1	92.8
Income (loss) before income taxes	(8.1)	(4.5)	(19.8)	(18.6)
Depreciation and amortization	$ 3.1	$ 2.5	$ 8.4	$ 7.4