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	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable, Impaired [Line Items]
Impaired loans with an allowance for loan loss required	$ 83,349	$ 36,084	$ 30,864
Impaired loans with no allowance for loan loss required	49,173	69,004	47,730
Total impaired loans	132,522	105,088	78,594
Allowance for loan losses related to impaired loans	14,365	8,023	7,218
Financing Receivable
Financing Receivable, Impaired [Line Items]
Allowance for loan losses related to impaired loans	3,900
TDRs	$ 66,219	$ 49,786	$ 33,183