Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 3,289.8
Marketable debt securities, Fair Value	3,242.5
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	4.5	6.2
Marketable equity securities, Fair Value	5.5	3.8
Total marketable securities, Cost	3,299.8	3,188.3
Total marketable securities, Unrealized Gains	1.5	27.3
Total marketable securities, Unrealized Losses	53.3	7.9
Total marketable securities, Fair Value	3,248.0	3,207.7
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	228.5	341.0
Marketable debt securities, Unrealized Gains	0.1	3.6
Marketable debt securities, Unrealized Losses	3.3	0.1
Marketable debt securities, Fair Value	225.3	344.5
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	449.9	417.7
Marketable debt securities, Unrealized Gains	0.1	4.2
Marketable debt securities, Unrealized Losses	6.2	0.3
Marketable debt securities, Fair Value	443.8	421.6
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	1,024.4	931.7
Marketable debt securities, Unrealized Gains	0.2	8.7
Marketable debt securities, Unrealized Losses	14.1	0.1
Marketable debt securities, Fair Value	1,010.5	940.3
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	483.5	433.9
Marketable debt securities, Unrealized Gains		2.5
Marketable debt securities, Unrealized Losses	12.9	0.2
Marketable debt securities, Fair Value	470.6	436.2
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	639.3	634.8
Marketable debt securities, Unrealized Gains	0.8	5.4
Marketable debt securities, Unrealized Losses	6.9	0.5
Marketable debt securities, Fair Value	633.2	639.7
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	252.0	212.8
Marketable debt securities, Unrealized Gains	0.1	1.3
Marketable debt securities, Unrealized Losses	3.5	0.3
Marketable debt securities, Fair Value	248.6	213.8
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	212.2	206.4
Marketable debt securities, Unrealized Gains	0.2	1.6
Marketable debt securities, Unrealized Losses	1.9	0.2
Marketable debt securities, Fair Value	$ 210.5	$ 207.8

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