Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenues	$ 18,997.0us-gaap_Revenues	$ 17,123.8us-gaap_Revenues	$ 17,050.5us-gaap_Revenues
Investment income	22.3us-gaap_InvestmentIncomeNet	28.6us-gaap_InvestmentIncomeNet	33.1us-gaap_InvestmentIncomeNet
Income before income taxes	2,017.6	1,695.0	1,628.9
Depreciation and amortization	917.7us-gaap_DepreciationAndAmortization	810.7us-gaap_DepreciationAndAmortization	700.9us-gaap_DepreciationAndAmortization
Expenditures for long-lived assets	1,462.2	1,771.7	1,799.0
Assets	20,618.8us-gaap_Assets	20,725.5us-gaap_Assets	18,627.8us-gaap_Assets
Cash and marketable securities	2,937.1	2,925.2	2,395.9
Other Segments
Segment Reporting Information [Line Items]
Net sales and revenues	121.3	123.8	152.7
Income before income taxes	(31.9)	(26.5)	(7.0)
Depreciation and amortization	11.8	10.2	10.6
Expenditures for long-lived assets	12.1	20.8	22.8
Assets	106.1	298.5	198.4
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	17,792.8	15,948.9	15,951.7
Income before income taxes	1,624.9	1,326.2	1,288.0
Depreciation and amortization	432.7	368.4	325.3
Expenditures for long-lived assets	526.9	840.5	855.9
Assets	8,701.5	9,095.4	7,832.3
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	1,204.2	1,174.9	1,098.8
Income before income taxes	370.4	340.2	307.8
Depreciation and amortization	485.0	442.3	375.6
Expenditures for long-lived assets	935.3	931.2	943.1
Assets	11,917.3	11,630.1	10,795.5
Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	15,330.4	13,627.7	13,797.1
Income before income taxes	1,160.1	936.7	920.4
Depreciation and amortization	415.0	352.9	308.8
Expenditures for long-lived assets	504.9	812.9	816.0
Assets	4,871.1	5,123.3	4,530.2
Trucks \| Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	14,594.0	13,002.9	13,131.5
Trucks \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(736.4)	(624.8)	(665.6)
Parts
Segment Reporting Information [Line Items]
Net sales and revenues	3,125.9	2,868.3	2,712.1
Income before income taxes	496.7	416.0	374.6
Depreciation and amortization	5.9	5.3	5.9
Expenditures for long-lived assets	9.9	6.8	17.1
Assets	787.2	748.4	707.8
Parts \| Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	3,077.5	2,822.2	2,667.5
Parts \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	$ (48.4)	$ (46.1)	$ (44.6)

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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