Acquired Banks Credit Quality Ratios (Detail)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Credit Quality Ratios, Acquired Banks [Line Items]
Non-performing loans to total loans	0.82%	1.02%	1.30%
Non-performing assets to total loans (including OREO)	1.79%	1.49%	1.84%
Non-performing assets to total assets	1.41%	1.00%	1.11%
Acquired Banks
Credit Quality Ratios, Acquired Banks [Line Items]
Non-performing loans to total loans	2.29%
Non-performing assets to total loans (including OREO)	11.54%
Non-performing assets to total assets	8.73%