Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Net sales and revenues	$ 4,776.1	$ 3,283.6
Investment income	8.9	8.0
Income (loss) before income taxes	479.0	286.3
Depreciation and amortization	169.6	162.4
Truck and Other
Segment Reporting Information [Line Items]
Net sales and revenues	4,514.7	3,042.6
Income (loss) before income taxes	398.8	228.0
Depreciation and amortization	81.9	80.6
Truck and Other | Trucks
Segment Reporting Information [Line Items]
Total sales and revenues	4,648.4	3,183.1
Less intersegment sales and revenues	(173.6)	(165.0)
Net sales and revenues	4,474.8	3,018.1
Income (loss) before income taxes	401.9	240.7
Depreciation and amortization	79.3	78.4
Truck and Other | Other products
Segment Reporting Information [Line Items]
Net sales and revenues	39.9	24.5
Income (loss) before income taxes	(3.1)	(12.7)
Depreciation and amortization	2.6	2.2
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	261.4	241.0
Income (loss) before income taxes	71.3	50.3
Depreciation and amortization	$ 87.7	$ 81.8