Marketable Debt Securities (Detail) (Truck, Parts and Other, USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,313.3	$ 1,264.9
Unrealized Gains	7.6	3.6
Unrealized Losses	0.3	1.0
Fair Value	1,320.6	1,267.5	[1]
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	359.0	214.9
Unrealized Gains	1.3	1.2
Unrealized Losses	0.1
Fair Value	360.2	216.1
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	61.1	78.2
Unrealized Gains	0.3	0.1
Unrealized Losses		0.1
Fair Value	61.4	78.2
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5.4	5.5
Fair Value	5.4	5.5
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	589.2	608.5
Unrealized Gains	4.1	1.2
Unrealized Losses		0.4
Fair Value	593.3	609.3
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	186.0	217.3
Unrealized Gains	1.7	0.7
Unrealized Losses	0.2	0.5
Fair Value	187.5	217.5
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	112.6	140.5
Unrealized Gains	0.2	0.4
Fair Value	$ 112.8	$ 140.9

[1] The December 31, 2013 consolidated balance sheet has been derived from audited financial statements.