Marketable Debt Securities (Detail) (Truck, Parts and Other, USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,264.8	$ 1,264.9
Unrealized Gains	7.6	3.6
Unrealized Losses	0.4	1.0
Fair Value	1,272.0	1,267.5
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	362.9	214.9
Unrealized Gains	0.8	1.2
Unrealized Losses	0.3
Fair Value	363.4	216.1
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	80.9	78.2
Unrealized Gains	0.6	0.1
Unrealized Losses		0.1
Fair Value	81.5	78.2
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8.0	5.5
Fair Value	8.0	5.5
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	528.1	608.5
Unrealized Gains	3.9	1.2
Unrealized Losses		0.4
Fair Value	532.0	609.3
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	192.1	217.3
Unrealized Gains	2.0	0.7
Unrealized Losses		0.5
Fair Value	194.1	217.5
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	92.8	140.5
Unrealized Gains	0.3	0.4
Unrealized Losses	0.1
Fair Value	$ 93.0	$ 140.9