Other Noninterest Income and Other Noninterest Expense - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other Non operating Income Expense [Line Items]
Merger related charges	$ 5.6	$ 27.0	$ 0.8