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Investment Securities - Narrative (Detail)
$ in Thousands
9 Months Ended
Sep. 30, 2018
USD ($)
Investment
Dec. 31, 2017
USD ($)
Investment
Debt Securities, Available-for-sale [Line Items]    
Investments in equity securities   $ 7,061
Carrying value of securities pledged to secure public deposits and other contractual obligations $ 362,000 345,100
Pledging requirements for credit derivatives and SWAP agreements - securities $ 300 $ 1,800
Number of investments in non-federal issuer representing more than 10% of shareholders' equity | Investment 0 0
Maximum investment in any single non-federal issuer representing shareholders' equity 10.00% 10.00%
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 14,418 $ 6,714
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year 114  
Money Market Mutual Funds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Investments in equity securities   5,985
U.S. Government Corporations and Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 140 90
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year 4  
Corporate Bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 7,708 3,459
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year 16  
Mortgage-Backed Securities [Domain]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 6,400  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year 114  
State and Political Subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 84 32
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year 9  
Collateralized Mortgage Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 108 82
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year 1  
Equity Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Investments in equity securities   1,076
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss   $ 0
Unrealized Gain on Securities $ 26  
Other Noninterest Income [Member] | Equity Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Net realized gain on investment securities $ 26