Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,154.0	$ 1,023.5
Unrealized Gains	8.6	1.4
Unrealized Losses	0.5	4.5
Fair Value	1,162.1	1,020.4
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	318.1	326.0
Unrealized Gains	2.2	0.3
Unrealized Losses	0.1	1.2
Fair Value	320.2	325.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	163.8	147.6
Unrealized Gains	1.9	0.2
Unrealized Losses	0.0	0.4
Fair Value	165.7	147.4
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	128.4	98.9
Unrealized Gains	0.9	0.2
Unrealized Losses	0.0	0.4
Fair Value	129.3	98.7
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	347.7	272.5
Unrealized Gains	2.3	0.4
Unrealized Losses	0.2	1.6
Fair Value	349.8	271.3
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	72.3	55.9
Unrealized Gains	0.2	0.1
Unrealized Losses	0.1	0.1
Fair Value	72.4	55.9
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	123.7	122.6
Unrealized Gains	1.1	0.2
Unrealized Losses	0.1	0.8
Fair Value	$ 124.7	$ 122.0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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