Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 5,079.9	$ 4,569.6	$ 9,912.6	$ 8,949.5
Investment income	5.3	5.5	10.4	11.3
Income (loss) before income taxes	652.2	481.0	1,213.9	887.5
Depreciation and amortization	227.3	227.7	448.4	447.5
Other Segments
Segment Reporting Information [Line Items]
Net sales and revenues	26.2	34.0	52.4	64.4
Income (loss) before income taxes	(9.7)	(2.5)	(20.1)	(11.7)
Depreciation and amortization	3.5	2.7	7.1	5.3
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	4,786.1	4,267.0	9,334.1	8,353.2
Income (loss) before income taxes	556.1	383.8	1,023.7	699.0
Depreciation and amortization	107.0	104.5	209.1	208.2
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	293.8	302.6	578.5	596.3
Income (loss) before income taxes	90.8	91.7	179.8	177.2
Depreciation and amortization	120.3	123.2	239.3	239.3
Trucks
Segment Reporting Information [Line Items]
Net sales and revenues	4,228.4	3,679.4	8,209.2	7,168.0
Income (loss) before income taxes	420.1	259.6	759.2	471.9
Depreciation and amortization	101.9	100.5	198.9	200.2
Trucks \| Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	3,983.4	3,455.0	7,752.5	6,784.2
Trucks \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	(245.0)	(224.4)	(456.7)	(383.8)
Parts
Segment Reporting Information [Line Items]
Net sales and revenues	788.4	790.1	1,552.8	1,527.3
Income (loss) before income taxes	145.7	126.7	284.6	238.8
Depreciation and amortization	1.6	1.3	3.1	2.7
Parts \| Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	776.5	778.0	1,529.2	1,504.6
Parts \| Intersegment
Segment Reporting Information [Line Items]
Net sales and revenues	$ (11.9)	$ (12.1)	$ (23.6)	$ (22.7)

The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production.

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