Acquired Banks Credit Quality Ratios (Detail)	Mar. 31, 2013	Dec. 31, 2012
Credit Quality Ratios, Acquired Banks [Line Items]
Non-performing loans to total loans	0.80%	0.82%
Non-performing assets to total loans (including OREO)	1.51%	1.79%
Non-performing assets to total assets	1.16%	1.41%
Acquired Banks
Credit Quality Ratios, Acquired Banks [Line Items]
Non-performing loans to total loans	2.26%	2.29%
Non-performing assets to total loans (including OREO)	9.85%	11.54%
Non-performing assets to total assets	8.87%	8.73%