Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,409.7	$ 1,154.0
Unrealized Gains	19.4	8.6
Unrealized Losses	0.1	0.5
Fair Value	1,429.0	1,162.1
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	388.1	318.1
Unrealized Gains	4.4	2.2
Unrealized Losses	0.1	0.1
Fair Value	392.4	320.2
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	229.6	163.8
Unrealized Gains	3.3	1.9
Fair Value	232.9	165.7
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	92.1	128.4
Unrealized Gains	2.3	0.9
Fair Value	94.4	129.3
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	406.0	347.7
Unrealized Gains	5.5	2.3
Unrealized Losses		0.2
Fair Value	411.5	349.8
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	77.0	72.3
Unrealized Gains	0.5	0.2
Unrealized Losses		0.1
Fair Value	77.5	72.4
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	216.9	123.7
Unrealized Gains	3.4	1.1
Unrealized Losses		0.1
Fair Value	$ 220.3	$ 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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