Marketable Debt Securities (Detail) (Truck, Parts and Other, USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,197.6	$ 1,183.5
Unrealized Gains	4.2	9.6
Unrealized Losses	0.7	0.4
Fair Value	1,201.1	1,192.7	[1]
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	205.4	217.2
Unrealized Gains	1.1	1.5
Unrealized Losses	0.1	0.1
Fair Value	206.4	218.6
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	79.6	59.8
Unrealized Gains	0.1	0.3
Fair Value	79.7	60.1
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0.6	0.8
Fair Value	0.6	0.8
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	564.6	447.5
Unrealized Gains	1.4	1.4
Unrealized Losses	0.4	0.2
Fair Value	565.6	448.7
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	243.6	349.3
Unrealized Gains	1.1	5.8
Unrealized Losses	0.2	0.1
Fair Value	244.5	355.0
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	103.8	108.9
Unrealized Gains	0.5	0.6
Fair Value	$ 104.3	$ 109.5

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