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Significant Unobservable Inputs used in Fair Value Measurement (Detail) - Mortgage servicing rights - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Fair value $ 2,729 $ 228,099 $ 243,389
Valuation Technique Discounted Cash Flow    
Weighted Average Constant Prepayment Rate [1] 9.81%    
Weighted Average Discount Rate 12.00%    
[1] Represents annualized loan repayment rate assumptions.