Carrying Amount and Fair Value of Financial Services Fixed-Rate Loans and Fixed-Rate Debt (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financial Services
Assets, Carrying Amount
Fixed-rate loans, Carrying Amount	$ 3,378.9	$ 3,361.7
Liabilities, Carrying Amount
Fixed-rate debt, Carrying Amount	3,484.2	3,277.2
Assets, Fair Value
Fixed-rate loans, Fair Value	3,434.8	3,434.8
Liabilities, Fair Value
Fixed-rate debt, Fair Value	3,549.9	3,350.5
Truck, Parts and Other
Liabilities, Carrying Amount
Fixed-rate debt, Carrying Amount	150.0	150.0
Liabilities, Fair Value
Fixed-rate debt, Fair Value	$ 158.2	$ 160.6