Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,173.8	$ 1,154.0	[1]
Unrealized Gains	20.6	8.6
Unrealized Losses	0.1	0.5
Fair Value	1,194.3	1,162.1
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	294.2	318.1
Unrealized Gains	4.8	2.2
Unrealized Losses		0.1
Fair Value	299.0	320.2
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	180.7	163.8
Unrealized Gains	3.7	1.9
Fair Value	184.4	165.7
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	102.9	128.4
Unrealized Gains	2.8	0.9
Fair Value	105.7	129.3
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	348.2	347.7
Unrealized Gains	5.5	2.3
Unrealized Losses	0.1	0.2
Fair Value	353.6	349.8
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	78.8	72.3
Unrealized Gains	0.5	0.2
Unrealized Losses		0.1
Fair Value	79.3	72.4
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	169.0	123.7
Unrealized Gains	3.3	1.1
Unrealized Losses		0.1
Fair Value	$ 172.3	$ 124.7

[1] The December 31, 2019 consolidated balance sheet has been derived from audited financial statements.