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Significant Accounting Policies and Basis of Presentation - Narrative (Detail)
 12 Months Ended
Jan. 01, 2020
USD ($)
Dec. 31, 2020
USD ($)
CompensationPlan
position
contract
Segment
Swap
Dec. 31, 2019
USD ($)
contract
Swap
Dec. 31, 2018
USD ($)
Dec. 31, 2014
Dec. 31, 2017
USD ($)
Derivative [Line Items]            
Average reserve balances required to be maintained with federal reserve bank   $ 0 $ 69,100,000      
Available-for-sale debt securities, accrued interest   4,200,000        
Net proceeds from (purchases of) FHLB, Federal Reserve Bank, and other restricted stock   12,846,000 5,471,000 $ 16,233,000    
Shareholders' equity   1,117,086,000 1,052,795,000 956,816,000   $ 920,964,000
Accrued interest receivable   $ 80,412,000 38,072,000      
Contractual payment of principal duration (days)   90 days        
Contractual terms for loans (months)   6 months        
Maximum loan to value ratio   80.00%        
Threshold period past due, writeoff   120 days        
Goodwill and other intangibles   $ 14,298,000 15,195,000 16,499,000    
FHLB, Federal Reserve Bank, and other restricted stock   71,368,000 84,214,000      
Federal Home Loan Bank stock   $ 46,100,000 60,800,000      
Percentage of tax position that will be realized or sustained upon examination   50.00%        
Number of share-based compensation plans | CompensationPlan   4        
Number of reportable segments (segment) | Segment   2        
Other   $ 8,767,000 11,380,000 10,723,000    
Net deferred tax (asset) liability     11,740,000      
Collaborative Arrangement            
Derivative [Line Items]            
Other   12,600,000 7,700,000 8,400,000    
Interest-only GNMA securities            
Derivative [Line Items]            
Interest-only GNMA securities     $ 16,272,000      
Net proceeds from (purchases of) FHLB, Federal Reserve Bank, and other restricted stock   15,400,000        
Other investments, realized gain   $ 1,000,000.0        
Accounting Standards Update 2016-13            
Derivative [Line Items]            
Initial allowance on loans purchased with credit deterioration $ 200,000          
Financing receivable, purchased with credit deterioration, discount 300,000          
Interest rate swaps | Not Designated as Hedging Instrument            
Derivative [Line Items]            
Number of interest rate swaps | Swap   155 140      
Derivative notional amount   $ 1,400,000,000 $ 1,400,000,000      
Interest rate swaps | Cash Flow Hedging | Designated as Hedging Instrument            
Derivative [Line Items]            
Number of interest rate swaps | contract   5 4      
Derivative notional amount   $ 1,100,000,000 $ 725,000,000.0      
Interest rate swaps | Fair Value Hedging | Designated as Hedging Instrument            
Derivative [Line Items]            
Number of interest rate swaps | position   24        
Derivative notional amount   $ 272,300,000        
Credit contracts | Not Designated as Hedging Instrument            
Derivative [Line Items]            
Derivative notional amount   $ 177,200,000 167,100,000      
ESPP            
Derivative [Line Items]            
Purchase price under employee stock purchase plan (percent)   85.00%     85.00%  
Purchase price under employee stock purchase plan, discount (percent)   15.00%     15.00%  
Retained Earnings            
Derivative [Line Items]            
Shareholders' equity   $ 438,581,000 381,519,000 316,651,000   258,076,000
Accumulated Other Comprehensive Income (Loss)            
Derivative [Line Items]            
Shareholders' equity   $ (5,764,000) (1,250,000) $ (22,663,000)   (359,000)
Cumulative Effect, Period of Adoption, Adjustment            
Derivative [Line Items]            
Shareholders' equity     (61,475,000)      
Net deferred tax (asset) liability (21,510,000)          
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings            
Derivative [Line Items]            
Shareholders' equity $ (61,500,000)   $ (61,475,000)      
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings | Accounting Standards Update 2016-01            
Derivative [Line Items]            
Shareholders' equity           1,041,000
Cumulative Effect, Period of Adoption, Adjustment | Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2016-01            
Derivative [Line Items]            
Shareholders' equity           $ (1,041,000)