Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 1,547.3
Marketable debt securities, Fair Value	1,545.7
Marketable equity securities, Cost	15.4
Marketable equity securities, Unrealized Gains	1.3
Marketable equity securities, Unrealized Losses	3.0
Marketable equity securities, Fair Value	13.7
Total marketable securities, Cost	1,562.7	$ 1,409.7
Total marketable securities, Unrealized Gains	6.4	19.4
Total marketable securities, Unrealized Losses	9.7	0.1
Total marketable securities, Fair Value	1,559.4	1,429.0
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	431.4	388.1
Marketable debt securities, Unrealized Gains	1.5	4.4
Marketable debt securities, Unrealized Losses	0.4	0.1
Marketable debt securities, Fair Value	432.5	392.4
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	208.3	229.6
Marketable debt securities, Unrealized Gains	0.6	3.3
Marketable debt securities, Unrealized Losses	1.5
Marketable debt securities, Fair Value	207.4	232.9
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	95.9	92.1
Marketable debt securities, Unrealized Gains	0.6	2.3
Marketable debt securities, Unrealized Losses	0.1
Marketable debt securities, Fair Value	96.4	94.4
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	443.9	406.0
Marketable debt securities, Unrealized Gains	1.4	5.5
Marketable debt securities, Unrealized Losses	2.5
Marketable debt securities, Fair Value	442.8	411.5
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	75.2	77.0
Marketable debt securities, Unrealized Gains	0.1	0.5
Marketable debt securities, Unrealized Losses	0.3
Marketable debt securities, Fair Value	75.0	77.5
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	292.6	216.9
Marketable debt securities, Unrealized Gains	0.9	3.4
Marketable debt securities, Unrealized Losses	1.9
Marketable debt securities, Fair Value	$ 291.6	$ 220.3

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