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Marketable Securities (Detail) - Truck, Parts and Other - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]    
Marketable debt securities, Cost $ 1,539.1  
Marketable debt securities, Fair Value 1,546.2  
Marketable equity securities, Cost 5.4  
Marketable equity securities, Unrealized Gains 0.0  
Marketable equity securities, Unrealized Losses 2.2  
Marketable equity securities, Fair Value 3.2  
Total marketable securities, Cost 1,544.5 $ 1,409.7
Total marketable securities, Unrealized Gains 9.2 19.4
Total marketable securities, Unrealized Losses 4.3 0.1
Total marketable securities, Fair Value 1,549.4 1,429.0 [1]
U.S. tax-exempt securities    
Schedule Of Available For Sale Securities [Line Items]    
Marketable debt securities, Cost 494.1 388.1
Marketable debt securities, Unrealized Gains 2.4 4.4
Marketable debt securities, Unrealized Losses 0.6 0.1
Marketable debt securities, Fair Value 495.9 392.4
U.S. corporate securities    
Schedule Of Available For Sale Securities [Line Items]    
Marketable debt securities, Cost 208.5 229.6
Marketable debt securities, Unrealized Gains 1.3 3.3
Marketable debt securities, Unrealized Losses 0.3  
Marketable debt securities, Fair Value 209.5 232.9
U.S. government and agency securities    
Schedule Of Available For Sale Securities [Line Items]    
Marketable debt securities, Cost 85.6 92.1
Marketable debt securities, Unrealized Gains 1.1 2.3
Marketable debt securities, Fair Value 86.7 94.4
Non-U.S. corporate securities    
Schedule Of Available For Sale Securities [Line Items]    
Marketable debt securities, Cost 435.1 406.0
Marketable debt securities, Unrealized Gains 2.4 5.5
Marketable debt securities, Unrealized Losses 0.8  
Marketable debt securities, Fair Value 436.7 411.5
Non-U.S. government securities    
Schedule Of Available For Sale Securities [Line Items]    
Marketable debt securities, Cost 64.0 77.0
Marketable debt securities, Unrealized Gains 0.1 0.5
Marketable debt securities, Unrealized Losses 0.1  
Marketable debt securities, Fair Value 64.0 77.5
Other debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Marketable debt securities, Cost 251.8 216.9
Marketable debt securities, Unrealized Gains 1.9 3.4
Marketable debt securities, Unrealized Losses 0.3  
Marketable debt securities, Fair Value $ 253.4 $ 220.3
[1] The December 31, 2020 consolidated balance sheet has been derived from audited financial statements.