Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Revenue	$ 17,123.8	$ 17,050.5	$ 16,355.2
Net sales and revenues	17,123.8	17,050.5	16,355.2
Investment income	28.6	33.1	38.2
Income before income taxes	1,695.0	1,628.9	1,506.9
Depreciation and amortization	810.7	700.9	673.8
Expenditures for long-lived assets	1,771.7	1,799.0	1,841.6
Assets	20,725.5	18,627.8	17,172.7
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	15,948.9	15,951.7	15,325.9
Income before income taxes	1,326.2	1,288.0	1,232.3
Depreciation and amortization	368.4	325.3	327.8
Expenditures for long-lived assets	840.5	855.9	907.3
Assets	9,095.4	7,832.3	7,771.3
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Revenue	13,627.7	13,797.1	13,359.2
Income before income taxes	936.7	920.4	864.7
Depreciation and amortization	352.9	308.8	311.8
Expenditures for long-lived assets	812.9	816.0	876.9
Assets	5,123.3	4,530.2	4,043.9
Truck, Parts and Other | Trucks | Parts and Trucks
Segment Reporting Information [Line Items]
Revenue	(624.8)	(665.6)	(728.5)
Truck, Parts and Other | Trucks | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	13,002.9	13,131.5	12,630.7
Truck, Parts and Other | Parts
Segment Reporting Information [Line Items]
Revenue	2,868.3	2,712.1	2,617.1
Income before income taxes	416.0	374.6	394.1
Depreciation and amortization	5.3	5.9	6.8
Expenditures for long-lived assets	6.8	17.1	2.2
Assets	748.4	707.8	641.4
Truck, Parts and Other | Parts | Parts and Trucks
Segment Reporting Information [Line Items]
Revenue	(46.1)	(44.6)	(40.1)
Truck, Parts and Other | Parts | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	2,822.2	2,667.5	2,577.0
Truck, Parts and Other | Other products
Segment Reporting Information [Line Items]
Net sales and revenues	123.8	152.7	118.2
Income before income taxes	(26.5)	(7.0)	(26.5)
Depreciation and amortization	10.2	10.6	9.2
Expenditures for long-lived assets	20.8	22.8	28.2
Assets	298.5	198.4	185.3
Truck, Parts and Other | Cash, Cash Equivalents and Marketable Securities
Segment Reporting Information [Line Items]
Assets	2,925.2	2,395.9	2,900.7
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	1,174.9	1,098.8	1,029.3
Income before income taxes	340.2	307.8	236.4
Depreciation and amortization	442.3	375.6	346.0
Expenditures for long-lived assets	931.2	943.1	934.3
Assets	$ 11,630.1	$ 10,795.5	$ 9,401.4