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Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Summary of Impaired Loans, Including Restructured Loans) (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Financing Receivable, Impaired [Line Items]      
Impaired loans with an allowance for loan loss required $ 39,956 $ 36,084 $ 29,037
Impaired loans with no allowance for loan loss required 80,346 69,004 50,281
Total impaired loans 120,302 105,088 79,318
Allowance for loan losses related to impaired loans 8,541 8,023 5,633
Financing Receivable      
Financing Receivable, Impaired [Line Items]      
Allowance for loan losses related to impaired loans 1,100    
TDRs $ 65,310 $ 49,786 $ 33,091