Marketable Debt Securities (Detail) (Truck, Parts and Other, USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,264.9	$ 1,183.5
Unrealized Gains	3.6	9.6
Unrealized Losses	1.0	0.4
Fair Value	1,267.5	1,192.7
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	214.9	217.2
Unrealized Gains	1.2	1.5
Unrealized Losses		0.1
Fair Value	216.1	218.6
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	78.2	59.8
Unrealized Gains	0.1	0.3
Unrealized Losses	0.1
Fair Value	78.2	60.1
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5.5	0.8
Fair Value	5.5	0.8
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	608.5	447.5
Unrealized Gains	1.2	1.4
Unrealized Losses	0.4	0.2
Fair Value	609.3	448.7
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	217.3	349.3
Unrealized Gains	0.7	5.8
Unrealized Losses	0.5	0.1
Fair Value	217.5	355.0
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	140.5	108.9
Unrealized Gains	0.4	0.6
Fair Value	$ 140.9	$ 109.5