Allowance for Loan Losses and Credit Quality Information - Summary of Loan Modifications By Type (Detail) - loan	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Modifications [Line Items]
Loan modification total	35	47
Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	1
Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan modification total	3	1
Commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	6	5
Construction
Financing Receivable, Modifications [Line Items]
Loan modification total	1	6
Residential
Financing Receivable, Modifications [Line Items]
Loan modification total	5	9
Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	20	25
Contractual payment reduction
Financing Receivable, Modifications [Line Items]
Loan modification total	20	4
Contractual payment reduction | Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Contractual payment reduction | Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan modification total	2	0
Contractual payment reduction | Commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	6	1
Contractual payment reduction | Construction
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Contractual payment reduction | Residential
Financing Receivable, Modifications [Line Items]
Loan modification total	4	2
Contractual payment reduction | Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	8	1
Maturity date extension
Financing Receivable, Modifications [Line Items]
Loan modification total	4	16
Maturity date extension | Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	1
Maturity date extension | Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan modification total	1	1
Maturity date extension | Commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	4
Maturity date extension | Construction
Financing Receivable, Modifications [Line Items]
Loan modification total	1	5
Maturity date extension | Residential
Financing Receivable, Modifications [Line Items]
Loan modification total	0	1
Maturity date extension | Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	2	4
Discharged in bankruptcy
Financing Receivable, Modifications [Line Items]
Loan modification total	8	17
Discharged in bankruptcy | Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Discharged in bankruptcy | Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Discharged in bankruptcy | Commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Discharged in bankruptcy | Construction
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Discharged in bankruptcy | Residential
Financing Receivable, Modifications [Line Items]
Loan modification total	1	5
Discharged in bankruptcy | Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	7	12
Other
Financing Receivable, Modifications [Line Items]
Loan modification total	3	10
Other | Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Other | Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Other | Commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Other | Construction
Financing Receivable, Modifications [Line Items]
Loan modification total	0	1
Other | Residential
Financing Receivable, Modifications [Line Items]
Loan modification total	0	1
Other | Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	3	8