Marketable Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	$ 1,603.1
Marketable debt securities, Fair Value	1,557.0
Marketable equity securities, Cost	15.4	$ 15.4
Marketable equity securities, Unrealized Gains		1.3
Marketable equity securities, Unrealized Losses	13.2	3.0
Marketable equity securities, Fair Value	2.2	13.7
Total marketable securities, Cost	1,618.5	1,562.7
Total marketable securities, Unrealized Gains	0.7	6.4
Total marketable securities, Unrealized Losses	60.0	9.7
Total marketable securities, Fair Value	1,559.2	1,559.4
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	469.7	431.4
Marketable debt securities, Unrealized Gains	0.3	1.5
Marketable debt securities, Unrealized Losses	7.0	0.4
Marketable debt securities, Fair Value	463.0	432.5
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	229.6	208.3
Marketable debt securities, Unrealized Gains	0.1	0.6
Marketable debt securities, Unrealized Losses	9.3	1.5
Marketable debt securities, Fair Value	220.4	207.4
U.S. taxable municipal/ provincial bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	167.0	192.2
Marketable debt securities, Unrealized Gains		0.6
Marketable debt securities, Unrealized Losses	8.2	1.2
Marketable debt securities, Fair Value	158.8	191.6
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	120.4	95.9
Marketable debt securities, Unrealized Gains	0.2	0.6
Marketable debt securities, Unrealized Losses	1.5	0.1
Marketable debt securities, Fair Value	119.1	96.4
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	440.3	443.9
Marketable debt securities, Unrealized Gains		1.4
Marketable debt securities, Unrealized Losses	14.7	2.5
Marketable debt securities, Fair Value	425.6	442.8
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	77.3	75.2
Marketable debt securities, Unrealized Gains	0.1	0.1
Marketable debt securities, Unrealized Losses	2.0	0.3
Marketable debt securities, Fair Value	75.4	75.0
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities, Cost	98.8	100.4
Marketable debt securities, Unrealized Gains		0.3
Marketable debt securities, Unrealized Losses	4.1	0.7
Marketable debt securities, Fair Value	$ 94.7	$ 100.0

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