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Derivative Instruments And Hedging Activities - Narrative (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
derivative
Swap
Dec. 31, 2016
USD ($)
derivative
Swap
Dec. 31, 2015
USD ($)
Derivative Instruments, Gain (Loss) [Line Items]      
Reclassification from AOCI to interest expense $ 300,000    
Period for hedging exposure to the variability in future cash flows for forecasted transactions 24 months    
Assets needed for immediate settlement of derivatives in a net liability position $ 4,400,000    
Minimum collateral with counterparties $ 4,900,000    
Not Designated as Hedging Instrument | Minimum      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative expiration period 30 days    
Not Designated as Hedging Instrument | Maximum      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative expiration period 60 days    
Not Designated as Hedging Instrument | Interest Rate Swap      
Derivative Instruments, Gain (Loss) [Line Items]      
Number of interest rate swaps | Swap 76 76  
Derivative notional amount $ 800,500,000.0 $ 716,600,000.0  
Not Designated as Hedging Instrument | Residential mortgage loan commitments      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative notional amount 2,700,000.0 3,600,000.0  
Not Designated as Hedging Instrument | Credit contracts      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative notional amount $ 80,500,000.0 $ 44,900,000.0  
Cash Flow Hedging | Designated as Hedging Instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Number of interest rate swaps | derivative   4  
Derivative notional amount   $ 325,000,000  
Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap      
Derivative Instruments, Gain (Loss) [Line Items]      
Number of interest rate swaps | derivative 9 4  
Derivative notional amount $ 550,000,000 $ 325,000,000  
Interest expense | Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of gain (loss) reclassified from accumulated OCI into income (effective portion) $ (2,634,000) $ (1,946,000) $ 0