Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans 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, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Impaired [Line Items]
Impaired loans with an allowance for loan loss required	[1]	$ 51,113	$ 69,487	$ 68,471
Impaired loans with no allowance for loan loss required		61,914	57,925	61,066
Total impaired loans	[2]	113,027	127,412	129,537
Allowance for loan losses related to impaired loans		8,483	6,270	6,577
Financing Receivable
Financing Receivable, Impaired [Line Items]
Allowance for loan losses related to impaired loans		3,400
TDRs		$ 59,320	$ 82,275	$ 83,385

[1] These impaired loans require an allowance for loan losses because the estimated fair value of the loans or related collateral is less than the recorded investment in the loans.

[2] Impaired loans are considered by the Company to be non-accrual loans, TDRs or loans with principal and/or interest at risk, even if the loan is current with all payments of principal and interest.