Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,394.1	$ 1,264.8
Unrealized Gains	6.1	7.6
Unrealized Losses	1.4	0.4
Fair Value	1,398.8	1,272.0	[1]
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	504.3	362.9
Unrealized Gains	2.1	0.8
Unrealized Losses	0.1	0.3
Fair Value	506.3	363.4
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77.4	80.9
Unrealized Gains	0.2	0.6
Unrealized Losses	0.1
Fair Value	77.5	81.5
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8.3	8.0
Unrealized Gains	0.1
Fair Value	8.4	8.0
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	568.1	528.1
Unrealized Gains	2.1	3.9
Unrealized Losses	1.2
Fair Value	569.0	532.0
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	168.2	192.1
Unrealized Gains	1.2	2.0
Fair Value	169.4	194.1
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67.8	92.8
Unrealized Gains	0.4	0.3
Unrealized Losses		0.1
Fair Value	$ 68.2	$ 93.0

[1] The December 31, 2014 consolidated balance sheet has been derived from audited financial statements.