Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Net sales and revenues	$ 3,924.3	$ 4,776.1
Investment income	6.5	8.9
Income before income taxes	337.2	479.0
Depreciation and amortization	184.7	170.6
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	3,631.2	4,514.7
Income before income taxes	250.6	398.8
Depreciation and amortization	81.5	81.9
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Total sales and revenues	3,075.8	3,971.7
Less intersegment	(142.5)	(177.3)
Income before income taxes	165.0	300.3
Depreciation and amortization	77.7	77.7
Truck, Parts and Other | Trucks | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	2,933.3	3,794.4
Truck, Parts and Other | Parts
Segment Reporting Information [Line Items]
Total sales and revenues	678.4	691.1
Less intersegment	(11.0)	(10.7)
Income before income taxes	95.3	101.6
Depreciation and amortization	1.3	1.6
Truck, Parts and Other | Parts | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	667.4	680.4
Truck, Parts and Other | Other products
Segment Reporting Information [Line Items]
Net sales and revenues	30.5	39.9
Income before income taxes	(9.7)	(3.1)
Depreciation and amortization	2.5	2.6
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	293.1	261.4
Income before income taxes	80.1	71.3
Depreciation and amortization	$ 103.2	$ 88.7