Regulatory Capital Matters and Capital Purchase Program - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				12 Months Ended				12 Months Ended		1 Months Ended
	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2012 Cumulative Preferred Stock Series D Fixed Rate Cumulative Perpetual Preferred Stock	Dec. 31, 2011 Cumulative Preferred Stock Series D Fixed Rate Cumulative Perpetual Preferred Stock	Dec. 31, 2012 Cumulative Preferred Stock Series D Fixed Rate Cumulative Perpetual Preferred Stock First Five Years	Dec. 31, 2012 Cumulative Preferred Stock Series D Fixed Rate Cumulative Perpetual Preferred Stock Thereafter	Dec. 31, 2012 Cumulative Preferred Stock Series E Fixed Rate Cumulative Perpetual Preferred Stock	Dec. 31, 2011 Cumulative Preferred Stock Series E Fixed Rate Cumulative Perpetual Preferred Stock	Dec. 31, 2012 Cumulative Preferred Stock Series E Fixed Rate Cumulative Perpetual Preferred Stock First Five Years	Dec. 31, 2012 Cumulative Preferred Stock Series E Fixed Rate Cumulative Perpetual Preferred Stock Thereafter	Nov. 14, 2008 Purchase Agreement Cumulative Preferred Stock Series D Fixed Rate Cumulative Perpetual Preferred Stock	Nov. 14, 2008 Purchase Agreement Cumulative Preferred Stock Series D Fixed Rate Cumulative Perpetual Preferred Stock First Five Years	Nov. 14, 2008 Purchase Agreement Cumulative Preferred Stock Series D Fixed Rate Cumulative Perpetual Preferred Stock Thereafter	Dec. 04, 2009 Purchase Agreement Cumulative Preferred Stock Series E Fixed Rate Cumulative Perpetual Preferred Stock	Dec. 04, 2009 Purchase Agreement Cumulative Preferred Stock Series E Fixed Rate Cumulative Perpetual Preferred Stock First Five Years	Dec. 04, 2009 Purchase Agreement Cumulative Preferred Stock Series E Fixed Rate Cumulative Perpetual Preferred Stock Thereafter
Regulatory Matters [Line Items]
Preferred non-voting stock, shares Issued			9,000	9,000			6,000	6,000			9,000			6,000
Preferred non-voting stock, par value			$ 0.01	$ 0.01			$ 0.01	$ 0.01			$ 0.01			$ 0.01
Preferred stock, liquidation preference per share			$ 1,000.00				$ 1,000.00				$ 1,000.00			$ 1,000.00
Value of stocks issued											$ 9.0			$ 6.0
Preferred stock dividend rates					5.00%	9.00%			5.00%	9.00%		5.00%	9.00%		5.00%	9.00%
Accumulated dividends		$ 2.1
Cash dividend per share	$ 0.05