Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 1,610.5
Marketable debt securities, Fair Value	1,541.7
Marketable equity securities, Cost	15.4	$ 15.4
Marketable equity securities, Unrealized Gains		1.3
Marketable equity securities, Unrealized Losses	12.9	3.0
Marketable equity securities, Fair Value	2.5	13.7
Total marketable securities, Cost	1,625.9	1,562.7
Total marketable securities, Unrealized Gains		6.4
Total marketable securities, Unrealized Losses	81.7	9.7
Total marketable securities, Fair Value	1,544.2	1,559.4
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	455.5	431.4
Marketable debt securities, Unrealized Gains		1.5
Marketable debt securities, Unrealized Losses	12.6	0.4
Marketable debt securities, Fair Value	442.9	432.5
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	249.3	208.3
Marketable debt securities, Unrealized Gains		0.6
Marketable debt securities, Unrealized Losses	13.8	1.5
Marketable debt securities, Fair Value	235.5	207.4
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	187.5	192.2
Marketable debt securities, Unrealized Gains		0.6
Marketable debt securities, Unrealized Losses	11.4	1.2
Marketable debt securities, Fair Value	176.1	191.6
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	112.3	95.9
Marketable debt securities, Unrealized Gains		0.6
Marketable debt securities, Unrealized Losses	3.3	0.1
Marketable debt securities, Fair Value	109.0	96.4
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	428.0	443.9
Marketable debt securities, Unrealized Gains		1.4
Marketable debt securities, Unrealized Losses	19.7	2.5
Marketable debt securities, Fair Value	408.3	442.8
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	73.1	75.2
Marketable debt securities, Unrealized Gains		0.1
Marketable debt securities, Unrealized Losses	3.0	0.3
Marketable debt securities, Fair Value	70.1	75.0
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	104.8	100.4
Marketable debt securities, Unrealized Gains		0.3
Marketable debt securities, Unrealized Losses	5.0	0.7
Marketable debt securities, Fair Value	$ 99.8	$ 100.0

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