Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net sales and revenues	$ 19,115.1	$ 18,997.0	$ 17,123.8
Investment income	21.8	22.3	28.6
Income before income taxes	2,337.1	2,017.6	1,695.0
Depreciation and amortization	907.1	917.7	810.7
Assets	21,109.8	20,618.8	20,725.5
Expenditures for long-lived assets	1,747.0	1,462.2	1,771.7
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	17,942.8	17,792.8	15,948.9
Income before income taxes	1,952.7	1,624.9	1,326.2
Cash and marketable securities	3,378.0	2,937.1	2,925.2
Depreciation and amortization	420.9	432.7	368.4
Assets	8,855.2	8,701.5	9,095.4
Expenditures for long-lived assets	702.6	526.9	840.5
Truck, Parts and Other \| Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	14,782.5	14,594.0	13,002.9
Income before income taxes	1,440.3	1,160.1	936.7
Depreciation and amortization	399.8	415.0	352.9
Assets	4,472.3	4,871.1	5,123.3
Expenditures for long-lived assets	660.0	504.9	812.9
Truck, Parts and Other \| Trucks \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	15,568.6	15,330.4	13,627.7
Truck, Parts and Other \| Trucks \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(786.1)	(736.4)	(624.8)
Truck, Parts and Other \| Parts
Segment Reporting Information [Line Items]
Net sales and revenues	3,060.1	3,077.5	2,822.2
Income before income taxes	555.6	496.7	416.0
Depreciation and amortization	6.2	5.9	5.3
Assets	793.3	787.2	748.4
Expenditures for long-lived assets	24.9	9.9	6.8
Truck, Parts and Other \| Parts \| Operating Segments
Segment Reporting Information [Line Items]
Net sales and revenues	3,104.7	3,125.9	2,868.3
Truck, Parts and Other \| Parts \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(44.6)	(48.4)	(46.1)
Truck, Parts and Other \| Other
Segment Reporting Information [Line Items]
Net sales and revenues	100.2	121.3	123.8
Income before income taxes	(43.2)	(31.9)	(26.5)
Depreciation and amortization	14.9	11.8	10.2
Assets	211.6	106.1	298.5
Expenditures for long-lived assets	17.7	12.1	20.8
Financial Services
Segment Reporting Information [Line Items]
Revenue	1,172.3	1,204.2	1,174.9
Income before income taxes	362.6	370.4	340.2
Depreciation and amortization	486.2	485.0	442.3
Assets	12,254.6	11,917.3	11,630.1
Expenditures for long-lived assets	$ 1,044.4	$ 935.3	$ 931.2

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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