Contractual Maturities of Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost Maturities:
Within one year	$ 279.3
One to five years	718.0
More than ten years	9.7
Amortized Cost	1,007.0	$ 1,370.2
Fair Value Maturities:
Within one year	278.6
One to five years	712.0
More than ten years	9.7
Marketable debt securities	$ 1,000.3	$ 1,367.1	[1]

[1] The December 31, 2017 consolidated balance sheet has been derived from audited financial statements.