Contractual Maturities of Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Amortized Cost Maturities:
Within one year	$ 437.1
One to five years	1,008.0
Six to ten years	0.2
Amortized Cost	1,445.3	$ 1,264.8
Fair Value Maturities:
Within one year	437.4
One to five years	1,010.5
Six to ten years	0.2
Fair Value	$ 1,448.1	$ 1,272.0