Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 3,256.1
Marketable debt securities, Fair Value	3,252.8
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	3.2	6.2
Marketable equity securities, Fair Value	6.8	3.8
Total marketable securities, Cost	3,266.1	3,188.3
Total marketable securities, Unrealized Gains	9.1	27.3
Total marketable securities, Unrealized Losses	15.6	7.9
Total marketable securities, Fair Value	3,259.6	3,207.7
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	269.1	341.0
Marketable debt securities, Unrealized Gains	2.3	3.6
Marketable debt securities, Unrealized Losses	0.1	0.1
Marketable debt securities, Fair Value	271.3	344.5
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	433.5	417.7
Marketable debt securities, Unrealized Gains	1.2	4.2
Marketable debt securities, Unrealized Losses	1.1	0.3
Marketable debt securities, Fair Value	433.6	421.6
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	979.5	931.7
Marketable debt securities, Unrealized Gains	2.3	8.7
Marketable debt securities, Unrealized Losses	3.6	0.1
Marketable debt securities, Fair Value	978.2	940.3
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	471.1	433.9
Marketable debt securities, Unrealized Gains		2.5
Marketable debt securities, Unrealized Losses	4.4	0.2
Marketable debt securities, Fair Value	466.7	436.2
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	630.4	634.8
Marketable debt securities, Unrealized Gains	2.2	5.4
Marketable debt securities, Unrealized Losses	1.8	0.5
Marketable debt securities, Fair Value	630.8	639.7
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	258.1	212.8
Marketable debt securities, Unrealized Gains	0.6	1.3
Marketable debt securities, Unrealized Losses	0.7	0.3
Marketable debt securities, Fair Value	258.0	213.8
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	214.4	206.4
Marketable debt securities, Unrealized Gains	0.5	1.6
Marketable debt securities, Unrealized Losses	0.7	0.2
Marketable debt securities, Fair Value	$ 214.2	$ 207.8

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