Marketable Debt Securities Continuous Unrealized Losses (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 579.0	$ 249.6
Less than 12 Months Unrealized Losses	1.1	0.4
12 Months or Greater Fair value	0.0	0.0
12 Months or Greater Unrealized losses	$ 0.0	$ 0.0