Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Net sales and revenues	$ 4,569.6	$ 4,300.5	$ 8,949.5	$ 8,224.8
Investment income	5.5	8.0	11.3	14.5
Income (loss) before income taxes	481.0	432.9	887.5	770.1
Depreciation and amortization	227.7	197.6	447.5	382.3
Trucks
Segment Reporting Information [Line Items]
Revenue	3,679.4	3,432.2	7,168.0	6,508.0
Income (loss) before income taxes	259.6	238.4	471.9	403.4
Depreciation and amortization	100.5	84.0	200.2	161.7
Trucks | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	3,455.0	3,270.0	6,784.2	6,203.3
Trucks | Intersegment
Segment Reporting Information [Line Items]
Revenue	(224.4)	(162.2)	(383.8)	(304.7)
Parts
Segment Reporting Information [Line Items]
Revenue	790.1	721.4	1,527.3	1,399.8
Income (loss) before income taxes	126.7	109.4	238.8	204.7
Depreciation and amortization	1.3	1.4	2.7	2.7
Parts | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	778.0	709.5	1,504.6	1,376.9
Parts | Intersegment
Segment Reporting Information [Line Items]
Revenue	(12.1)	(11.9)	(22.7)	(22.9)
Other products
Segment Reporting Information [Line Items]
Net sales and revenues	34.0	32.2	64.4	62.7
Income (loss) before income taxes	(2.5)	(4.4)	(11.7)	(14.1)
Depreciation and amortization	2.7	2.4	5.3	4.9
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	4,267.0	4,011.7	8,353.2	7,642.9
Income (loss) before income taxes	383.8	343.4	699.0	594.0
Depreciation and amortization	104.5	87.8	208.2	169.3
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	302.6	288.8	596.3	581.9
Income (loss) before income taxes	91.7	81.5	177.2	161.6
Depreciation and amortization	$ 123.2	$ 109.8	$ 239.3	$ 213.0