Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,052.2	$ 1,370.2
Unrealized Gains	0.7	1.9
Unrealized Losses	6.4	5.0
Fair Value	1,046.5	1,367.1	[1]
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	523.6	537.9
Unrealized Losses	2.5	2.4
Fair Value	521.1	535.5
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	50.5	89.7
Unrealized Gains	0.1	0.2
Unrealized Losses	0.4	0.2
Fair Value	50.2	89.7
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	59.1	48.9
Unrealized Losses	0.4	0.2
Fair Value	58.7	48.7
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	246.9	459.4
Unrealized Gains	0.5	1.3
Unrealized Losses	1.9	1.4
Fair Value	245.5	459.3
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	53.4	91.5
Unrealized Gains	0.1	0.3
Unrealized Losses	0.1	0.1
Fair Value	53.4	91.7
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	118.7	142.8
Unrealized Gains		0.1
Unrealized Losses	1.1	0.7
Fair Value	$ 117.6	$ 142.2

[1] The December 31, 2017 consolidated balance sheet has been derived from audited financial statements.