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Fair Value Measurement - Quantitative Information about Recurring Level 3 Fair Value Measurements (Detail) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2015
Fair Value Inputs, Assets, Quantitative Information [Line Items]            
Constant prepayment rate 9.20% 12.80%        
Discount rate 14.00% 12.00%        
Mortgage Servicing Rights [Member]            
Fair Value Inputs, Assets, Quantitative Information [Line Items]            
Mortgages Servicing Rights, Fair Value $ 6,419 $ 6,208 $ 6,595 $ 6,596 $ 6,720 $ 7,618
Minimum [Member] | Mortgage Servicing Rights [Member]            
Fair Value Inputs, Assets, Quantitative Information [Line Items]            
Discount rate 14.00%   14.00%      
Minimum [Member] | Mortgage Servicing Rights [Member] | Discounted Cash Flow [Member]            
Fair Value Inputs, Assets, Quantitative Information [Line Items]            
Constant prepayment rate 6.50%   6.90%      
Maximum [Member] | Mortgage Servicing Rights [Member]            
Fair Value Inputs, Assets, Quantitative Information [Line Items]            
Discount rate 16.00%   16.00%      
Maximum [Member] | Mortgage Servicing Rights [Member] | Discounted Cash Flow [Member]            
Fair Value Inputs, Assets, Quantitative Information [Line Items]            
Constant prepayment rate 21.40%   16.60%      
Weighted Average [Member] | Mortgage Servicing Rights [Member]            
Fair Value Inputs, Assets, Quantitative Information [Line Items]            
Discount rate 14.00%   14.00%      
Weighted Average [Member] | Mortgage Servicing Rights [Member] | Discounted Cash Flow [Member]            
Fair Value Inputs, Assets, Quantitative Information [Line Items]            
Constant prepayment rate 9.20%   8.80%