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Share-Based Payment Arrangements - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Bank
Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Intrinsic value of options exercised $ 0.1 $ 0.4
Unrecognized compensation cost 36.4  
Weighted-average period in years 3 years 6 months  
Total fair value of shares vested $ 0.5 $ 0.7
Shareholder return period 3 years  
Number of peer group of regional banks 50  
Performance Share Award [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares granted to key members of executives and senior management 67,533  
Fair Value of shares granted $ 32.84  
Percentage of maximum number of shares earned of target award 200.00%