Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,214.4	$ 1,141.9
Unrealized Gains	3.4	2.7
Unrealized Losses	1.8	3.7
Fair Value	1,216.0	1,140.9	[1]
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	520.8	597.9
Unrealized Gains	1.3	0.2
Unrealized Losses	0.1	3.1
Fair Value	522.0	595.0
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58.6	47.6
Unrealized Gains	0.3	0.2
Unrealized Losses	0.0	0.0
Fair Value	58.9	47.8
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12.2	16.0
Unrealized Gains	0.0	0.0
Unrealized Losses	0.0	0.0
Fair Value	12.2	16.0
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	397.7	306.9
Unrealized Gains	1.4	1.5
Unrealized Losses	1.2	0.4
Fair Value	397.9	308.0
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	97.0	97.6
Unrealized Gains	0.3	0.6
Unrealized Losses	0.1	0.0
Fair Value	97.2	98.2
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	128.1	75.9
Unrealized Gains	0.1	0.2
Unrealized Losses	0.4	0.2
Fair Value	$ 127.8	$ 75.9

[1] The December 31, 2016 consolidated balance sheet has been derived from audited financial statements.