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Marketable Debt Securities Continuous Unrealized Losses (Detail) - Truck, Parts and Other - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Less than 12 Months Fair value $ 710.3 $ 908.5
Less than 12 Months Unrealized losses 4.7 4.8
12 Months or Greater Fair value 82.4 18.4
12 Months or Greater Unrealized losses $ 1.2 $ 0.2