Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,445.3	$ 1,264.8
Unrealized Gains	3.9	7.6
Unrealized Losses	1.1	0.4
Fair Value	1,448.1	1,272.0
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	505.0	362.9
Unrealized Gains	0.7	0.8
Unrealized Losses	0.3	0.3
Fair Value	505.4	363.4
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	76.7	80.9
Unrealized Gains	0.1	0.6
Unrealized Losses	0.1
Fair Value	76.7	81.5
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15.7	8.0
Unrealized Gains	0.1
Unrealized Losses	0.1
Fair Value	15.7	8.0
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	585.6	528.1
Unrealized Gains	1.8	3.9
Unrealized Losses	0.4
Fair Value	587.0	532.0
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	192.7	192.1
Unrealized Gains	1.1	2.0
Unrealized Losses	0.1
Fair Value	193.7	194.1
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69.6	92.8
Unrealized Gains	0.1	0.3
Unrealized Losses	0.1	0.1
Fair Value	$ 69.6	$ 93.0