Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Net sales and revenues	$ 3,820.2	$ 4,257.1	$ 13,053.5	$ 11,501.4
Investment income	7.8	11.0	24.9	28.9
Income (loss) before income taxes	345.4	394.7	1,270.7	1,034.2
Depreciation and amortization	176.2	174.3	522.4	505.7
Truck and Other
Segment Reporting Information [Line Items]
Net sales and revenues	3,546.7	3,993.0	12,252.5	10,738.3
Income (loss) before income taxes	257.2	321.9	1,016.7	836.3
Depreciation and amortization	79.1	81.5	245.6	243.5
Truck and Other | Trucks
Segment Reporting Information [Line Items]
Total sales and revenues	3,662.5	4,145.1	12,694.6	11,275.5
Less intersegment sales and revenues	(152.5)	(183.9)	(559.9)	(621.8)
Net sales and revenues	3,510.0	3,961.2	12,134.7	10,653.7
Income (loss) before income taxes	258.9	324.3	1,022.8	855.3
Depreciation and amortization	76.5	78.8	237.9	236.3
Truck and Other | Other products
Segment Reporting Information [Line Items]
Net sales and revenues	36.7	31.8	117.8	84.6
Income (loss) before income taxes	(1.7)	(2.4)	(6.1)	(19.0)
Depreciation and amortization	2.6	2.7	7.7	7.2
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	273.5	264.1	801.0	763.1
Income (loss) before income taxes	80.4	61.8	229.1	169.0
Depreciation and amortization	$ 97.1	$ 92.8	$ 276.8	$ 262.2