Loans and the Allowance for Credit Losses - Recorded Investment in Non-Accrual Loans and TDRs (Parenthetical) (Detail)	Jun. 30, 2018 USD ($) Borrowers	Dec. 31, 2017 USD ($) Borrowers
Receivables [Abstract]
Number of borrowers with loans greater than one million | Borrowers	2	3
Borrowers with large amount of loans outstanding, minimum amount of loans per borrower	$ 1,000,000	$ 1,000,000
Borrowers with large amount of loans outstanding, net	$ 5,200,000	$ 6,800,000