Allowance for Loan and Lease Losses and Credit Quality Information - Summary of Loan Modifications By Type (Detail) - loan	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Modifications [Line Items]
Loan modification total	40	35
Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	2	0
Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan modification total	4	3
Commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	3	6
Construction
Financing Receivable, Modifications [Line Items]
Loan modification total	0	1
Residential
Financing Receivable, Modifications [Line Items]
Loan modification total	10	5
Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	21	20
Contractual payment reduction
Financing Receivable, Modifications [Line Items]
Loan modification total	12	20
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	3	2
Contractual payment reduction | Commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	6
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	4
Contractual payment reduction | Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	5	8
Maturity date extension
Financing Receivable, Modifications [Line Items]
Loan modification total	4	4
Maturity date extension | Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Maturity date extension | Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan modification total	0	1
Maturity date extension | Commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	1	0
Maturity date extension | Construction
Financing Receivable, Modifications [Line Items]
Loan modification total	0	1
Maturity date extension | Residential
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Maturity date extension | Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	3	2
Discharged in bankruptcy
Financing Receivable, Modifications [Line Items]
Loan modification total	11	8
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	3	1
Discharged in bankruptcy | Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	8	7
Other
Financing Receivable, Modifications [Line Items]
Loan modification total	13	3
Other | Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	2	0
Other | Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan modification total	1	0
Other | Commercial
Financing Receivable, Modifications [Line Items]
Loan modification total	2	0
Other | Construction
Financing Receivable, Modifications [Line Items]
Loan modification total	0	0
Other | Residential
Financing Receivable, Modifications [Line Items]
Loan modification total	3	0
Other | Consumer
Financing Receivable, Modifications [Line Items]
Loan modification total	5	3