Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,408.4	$ 1,409.7	[1]
Unrealized Gains	14.1	19.4
Unrealized Losses	1.4	0.1
Fair Value	1,421.1	1,429.0	[1]
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	395.0	388.1
Unrealized Gains	3.5	4.4
Unrealized Losses	0.2	0.1
Fair Value	398.3	392.4
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	226.2	229.6
Unrealized Gains	2.0	3.3
Unrealized Losses	0.3
Fair Value	227.9	232.9
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	91.9	92.1
Unrealized Gains	1.8	2.3
Fair Value	93.7	94.4
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	401.3	406.0
Unrealized Gains	3.9	5.5
Unrealized Losses	0.6
Fair Value	404.6	411.5
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	74.3	77.0
Unrealized Gains	0.2	0.5
Unrealized Losses	0.1
Fair Value	74.4	77.5
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	219.7	216.9
Unrealized Gains	2.7	3.4
Unrealized Losses	0.2
Fair Value	$ 222.2	$ 220.3

[1] The December 31, 2020 consolidated balance sheet has been derived from audited financial statements.