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	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired loans with an allowance for loan loss required	$ 37,572	$ 36,084	$ 39,968
Impaired loans with no allowance for loan loss required	65,559	69,004	47,554
Total impaired loans	103,131	105,088	87,522
Allowance for loan losses related to impaired loans	6,863	8,023	8,165
Financing Receivable
Financing Receivable, Impaired [Line Items]
Allowance for loan losses related to impaired loans	1,100
TDRs	$ 47,676	$ 49,786	$ 39,669