Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,091.0	$ 1,154.0
Unrealized Gains	10.1	8.6
Unrealized Losses	4.1	0.5
Fair Value	1,097.0	1,162.1
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	306.1	318.1
Unrealized Gains	1.6	2.2
Unrealized Losses	0.8	0.1
Fair Value	306.9	320.2
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	157.9	163.8
Unrealized Gains	1.3	1.9
Unrealized Losses	0.6
Fair Value	158.6	165.7
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	119.4	128.4
Unrealized Gains	4.0	0.9
Fair Value	123.4	129.3
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	299.8	347.7
Unrealized Gains	1.0	2.3
Unrealized Losses	2.6	0.2
Fair Value	298.2	349.8
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	70.8	72.3
Unrealized Gains	0.4	0.2
Unrealized Losses		0.1
Fair Value	71.2	72.4
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	137.0	123.7
Unrealized Gains	1.8	1.1
Unrealized Losses	0.1	0.1
Fair Value	$ 138.7	$ 124.7

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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