Marketable Debt Securities (Detail) (Truck, Parts and Other, USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,168.8	$ 1,183.5
Unrealized Gains	9.3	9.6
Unrealized Losses	0.4	0.4
Fair Value	1,177.7	1,192.7	[1]
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	219.7	217.2
Unrealized Gains	1.7	1.5
Unrealized Losses		0.1
Fair Value	221.4	218.6
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65.4	59.8
Unrealized Gains	0.3	0.3
Fair Value	65.7	60.1
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0.4	0.8
Unrealized Gains	0.1
Fair Value	0.5	0.8
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	333.0	349.3
Unrealized Gains	5.4	5.8
Unrealized Losses		0.1
Fair Value	338.4	355.0
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	437.6	447.5
Unrealized Gains	1.3	1.4
Unrealized Losses	0.3	0.2
Fair Value	438.6	448.7
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	112.7	108.9
Unrealized Gains	0.5	0.6
Unrealized Losses	0.1
Fair Value	$ 113.1	$ 109.5

[1] The December 31, 2012 consolidated balance sheet has been derived from audited financial statements.