Residential Mortgage-Banking Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Compliance with Regulatory Capital Requirements for Mortgage Companies [Line Items]
Residential mortgage loans serviced for the benefit of others	$ 386.0	$ 388.7
Book value of residential mortgage-servicing rights	2.6	2.6
Estimated fair value of residential mortgage-servicing rights	$ 3.2	$ 3.1