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Securities - Additional Information (Detail) (USD $)
 6 Months Ended 12 Months Ended
Jun. 30, 2013
Contract
Dec. 31, 2012
Contract
Amortized Cost And Fair Value Debt Securities [Abstract]    
Number of U.S. agency and mortgage-backed securities 36 12
Number of obligations of state and political subdivisions in an unrealized loss position 18 9
Percentage of investment portfolio 100.00% 100.00%
Weighted-average period of re-pricing of portfolio 4 years 6 months 0 days 3 years 7 months 6 days
Impairment recognized $ 0