Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,141.9	$ 1,445.3
Unrealized Gains	2.7	3.9
Unrealized Losses	3.7	1.1
Fair Value	1,140.9	1,448.1
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	597.9	505.0
Unrealized Gains	0.2	0.7
Unrealized Losses	3.1	0.3
Fair Value	595.0	505.4
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47.6	76.7
Unrealized Gains	0.2	0.1
Unrealized Losses	0.0	0.1
Fair Value	47.8	76.7
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16.0	15.7
Unrealized Gains	0.0	0.1
Unrealized Losses	0.0	0.1
Fair Value	16.0	15.7
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	306.9	585.6
Unrealized Gains	1.5	1.8
Unrealized Losses	0.4	0.4
Fair Value	308.0	587.0
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	97.6	192.7
Unrealized Gains	0.6	1.1
Unrealized Losses	0.0	0.1
Fair Value	98.2	193.7
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75.9	69.6
Unrealized Gains	0.2	0.1
Unrealized Losses	0.2	0.1
Fair Value	$ 75.9	$ 69.6