Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 2,914.4
Marketable debt securities, Fair Value	2,941.1
Marketable equity securities, Cost	10.0	$ 10.0
Marketable equity securities, Unrealized Losses	4.6	3.7
Marketable equity securities, Fair Value	5.4	6.3
Total marketable securities, Cost	2,924.4	2,783.5
Total marketable securities, Unrealized Gains	28.4	10.7
Total marketable securities, Unrealized Losses	6.3	15.4
Total marketable securities, Fair Value	2,946.5	2,778.8
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	331.2	304.5
Marketable debt securities, Unrealized Gains	4.8	0.6
Marketable debt securities, Unrealized Losses	0.1	1.4
Marketable debt securities, Fair Value	335.9	303.7
U.S. taxable municipal / non-U.S. provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	389.4	381.1
Marketable debt securities, Unrealized Gains	4.2	1.2
Marketable debt securities, Unrealized Losses	0.3	2.2
Marketable debt securities, Fair Value	393.3	380.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	909.4	864.3
Marketable debt securities, Unrealized Gains	8.7	3.1
Marketable debt securities, Unrealized Losses	0.4	3.2
Marketable debt securities, Fair Value	917.7	864.2
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	299.4	287.1
Marketable debt securities, Unrealized Gains	2.0	0.1
Marketable debt securities, Unrealized Losses	0.2	1.5
Marketable debt securities, Fair Value	301.2	285.7
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	596.0	606.6
Marketable debt securities, Unrealized Gains	5.9	3.5
Marketable debt securities, Unrealized Losses	0.3	2.0
Marketable debt securities, Fair Value	601.6	608.1
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	203.1	163.5
Marketable debt securities, Unrealized Gains	1.2	1.4
Marketable debt securities, Unrealized Losses	0.2	0.6
Marketable debt securities, Fair Value	204.1	164.3
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	185.9	166.4
Marketable debt securities, Unrealized Gains	1.6	0.8
Marketable debt securities, Unrealized Losses	0.2	0.8
Marketable debt securities, Fair Value	$ 187.3	$ 166.4

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