Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,417.7	$ 1,445.3
Unrealized Gains	8.8	3.9
Unrealized Losses	0.2	1.1
Fair Value	1,426.3	1,448.1	[1]
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	490.3	505.0
Unrealized Gains	2.7	0.7
Unrealized Losses	0.0	0.3
Fair Value	493.0	505.4
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	100.9	76.7
Unrealized Gains	0.5	0.1
Unrealized Losses	0.1	0.1
Fair Value	101.3	76.7
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16.2	15.7
Unrealized Gains	0.3	0.1
Unrealized Losses	0.0	0.1
Fair Value	16.5	15.7
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	554.6	585.6
Unrealized Gains	2.9	1.8
Unrealized Losses	0.1	0.4
Fair Value	557.4	587.0
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	154.1	192.7
Unrealized Gains	1.8	1.1
Unrealized Losses	0.0	0.1
Fair Value	155.9	193.7
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	101.6	69.6
Unrealized Gains	0.6	0.1
Unrealized Losses	0.0	0.1
Fair Value	$ 102.2	$ 69.6

[1] The December 31, 2015 consolidated balance sheet has been derived from audited financial statements.