Loans and the Allowance for Credit Losses - Recorded Investment in Non-Accrual Loans and TDRs (Parenthetical) (Detail)	Jun. 30, 2019 USD ($) Borrower	Dec. 31, 2018 USD ($) Borrower
Receivables [Abstract]
Number of borrowers with loans greater than one million | Borrower	3	1
Borrowers with large amount of loans outstanding, minimum amount of loans per borrower	$ 1,000,000.0	$ 1,000,000.0
Borrowers with large amount of loans outstanding, net	$ 8,200,000	$ 3,400,000