Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,078.5	$ 1,023.5
Unrealized Gains	4.8	1.4
Unrealized Losses	1.7	4.5
Fair Value	1,081.6	1,020.4	[1]
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	304.2	326.0
Unrealized Gains	1.1	0.3
Unrealized Losses	0.4	1.2
Fair Value	304.9	325.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	162.4	147.6
Unrealized Gains	1.1	0.2
Unrealized Losses	0.1	0.4
Fair Value	163.4	147.4
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	110.9	98.9
Unrealized Gains	0.3	0.2
Unrealized Losses	0.2	0.4
Fair Value	111.0	98.7
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	301.8	272.5
Unrealized Gains	1.5	0.4
Unrealized Losses	0.5	1.6
Fair Value	302.8	271.3
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	74.2	55.9
Unrealized Gains	0.2	0.1
Unrealized Losses		0.1
Fair Value	74.4	55.9
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	125.0	122.6
Unrealized Gains	0.6	0.2
Unrealized Losses	0.5	0.8
Fair Value	$ 125.1	$ 122.0

[1] The December 31, 2018 consolidated balance sheet has been derived from audited financial statements.