Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]
Net sales and revenues	$ 4,379.9	$ 3,924.3
Investment income	5.8	6.5
Income (loss) before income taxes	406.5	337.2
Depreciation and amortization	219.8	184.7
Trucks
Segment Reporting Information [Line Items]
Revenue	3,488.6	3,075.8
Income (loss) before income taxes	212.3	165.0
Depreciation and amortization	99.7	77.7
Trucks | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	3,329.2	2,933.3
Trucks | Intersegment
Segment Reporting Information [Line Items]
Revenue	(159.4)	(142.5)
Parts
Segment Reporting Information [Line Items]
Revenue	737.2	678.4
Income (loss) before income taxes	112.1	95.3
Depreciation and amortization	1.4	1.3
Parts | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	726.6	667.4
Parts | Intersegment
Segment Reporting Information [Line Items]
Revenue	(10.6)	(11.0)
Other products
Segment Reporting Information [Line Items]
Net sales and revenues	30.4	30.5
Income (loss) before income taxes	(9.2)	(9.7)
Depreciation and amortization	2.6	2.5
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	4,086.2	3,631.2
Income (loss) before income taxes	315.2	250.6
Depreciation and amortization	103.7	81.5
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	293.7	293.1
Income (loss) before income taxes	85.5	80.1
Depreciation and amortization	$ 116.1	$ 103.2