Marketable Debt Securities (Detail) (Truck, Parts and Other, USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,328.7	$ 1,264.9
Unrealized Gains	6.8	3.6
Unrealized Losses	0.5	1.0
Fair Value	1,335.0	1,267.5	[1]
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	328.8	214.9
Unrealized Gains	1.5	1.2
Fair Value	330.3	216.1
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60.0	78.2
Unrealized Gains	0.2	0.1
Unrealized Losses		0.1
Fair Value	60.2	78.2
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5.5	5.5
Unrealized Gains	0.1
Fair Value	5.6	5.5
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	647.4	608.5
Unrealized Gains	3.4	1.2
Unrealized Losses	0.1	0.4
Fair Value	650.7	609.3
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	200.4	217.3
Unrealized Gains	1.5	0.7
Unrealized Losses	0.4	0.5
Fair Value	201.5	217.5
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	86.6	140.5
Unrealized Gains	0.1	0.4
Fair Value	$ 86.7	$ 140.9

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