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Impairment of Investment Securities - Additional Information (Detail)
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Debt and Equity Securities, FV-NI [Line Items]          
Other than temporary impairment charges recognized     $ 0 $ 0  
Amortized Cost $ 915,350,000   915,350,000   $ 740,846,000
Securities available for sale, at fair value 889,056,000   889,056,000   731,358,000
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss 43,987,000   $ 43,987,000   18,534,000
Probability of default     100.00%    
Federal Home Loan Bank Stock $ 25,029,000   $ 25,029,000   29,837,000
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total 183   183    
Pooled Trust Preferred Collateralized Debt Obligations [Member]          
Debt and Equity Securities, FV-NI [Line Items]          
Securities available for sale, at fair value         23,646,000
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss         4,379,000
Trust preferred collateralized debt obligations [Member]          
Debt and Equity Securities, FV-NI [Line Items]          
Other than temporary impairment charges recognized       $ 0  
Corporate Debt Securities [Member]          
Debt and Equity Securities, FV-NI [Line Items]          
Securities available for sale, at fair value $ 21,101,000   $ 21,101,000   16,493,000
Pooled Trust Preferred Securities [Member]          
Debt and Equity Securities, FV-NI [Line Items]          
Other than temporary impairment charges recognized   $ 0      
Equity Securities [Member]          
Debt and Equity Securities, FV-NI [Line Items]          
Other than temporary impairment charges recognized 0 $ 0      
Continuous Unrealized Loss Position, Fair Value $ 0   $ 0   0
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]          
Debt and Equity Securities, FV-NI [Line Items]          
Percent of Unrealized Losses 84.00%   84.00%    
Reported Value Measurement [Member]          
Debt and Equity Securities, FV-NI [Line Items]          
Securities available for sale, at fair value $ 889,056,000   $ 889,056,000   731,358,000
Federal Home Loan Bank Stock 25,029,000   25,029,000    
Pooled Trust Preferred Collateralized Debt Obligations [Member]          
Debt and Equity Securities, FV-NI [Line Items]          
Amortized Cost 0   0   27,499,000
Securities available for sale, at fair value $ 0   $ 0   $ 23,646,000