Contractual Maturities of Marketable Debt Securities (Detail) (Truck, Parts and Other, USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Truck, Parts and Other
Amortized Cost Maturities:
Within one year	$ 508.8
One to five years	659.8
Six to ten years	0.2
Amortized Cost	1,168.8	1,183.5
Fair Value Maturities:
Within one year	512.0
One to five years	665.5
Six to ten years	0.2
Fair Value	$ 1,177.7	$ 1,192.7	[1]

[1] The December 31, 2012 consolidated balance sheet has been derived from audited financial statements.