Carrying Amount and Fair Value of Financial Services Fixed-Rate Loans and Fixed-Rate Debt (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Carrying Amount | Financial Services
Assets:
Fixed-rate loans	$ 2,911.0	$ 2,740.1
Liabilities:
Fixed-rate debt	2,323.9	1,958.6
Carrying Amount | Truck and Other
Liabilities:
Fixed-rate debt	150.0	150.0
Fair Value | Financial Services
Assets:
Fixed-rate loans	2,953.0	2,776.1
Liabilities:
Fixed-rate debt	2,412.0	2,021.1
Fair Value | Truck and Other
Liabilities:
Fixed-rate debt	$ 166.5	$ 167.6