Allowance for Loan Losses and Credit Quality Information - Summary of Loan Modifications By Type (Detail) - loan	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Modifications [Line Items]
Loan Modification Total	47	27
Commercial
Financing Receivable, Modifications [Line Items]
Loan Modification Total	5	3
Construction
Financing Receivable, Modifications [Line Items]
Loan Modification Total	6	0
Residential
Financing Receivable, Modifications [Line Items]
Loan Modification Total	9	8
Consumer
Financing Receivable, Modifications [Line Items]
Loan Modification Total	25	14
Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan Modification Total	1	0
Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan Modification Total	1	2
Contractual payment reduction
Financing Receivable, Modifications [Line Items]
Loan Modification Total	4	12
Contractual payment reduction | Commercial
Financing Receivable, Modifications [Line Items]
Loan Modification Total	1	0
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	2	0
Contractual payment reduction | Consumer
Financing Receivable, Modifications [Line Items]
Loan Modification Total	1	12
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	0	0
Maturity date extension
Financing Receivable, Modifications [Line Items]
Loan Modification Total	16	4
Maturity date extension | Commercial
Financing Receivable, Modifications [Line Items]
Loan Modification Total	4	2
Maturity date extension | Construction
Financing Receivable, Modifications [Line Items]
Loan Modification Total	5	0
Maturity date extension | Residential
Financing Receivable, Modifications [Line Items]
Loan Modification Total	1	0
Maturity date extension | Consumer
Financing Receivable, Modifications [Line Items]
Loan Modification Total	4	0
Maturity date extension | Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Loan Modification Total	1	0
Maturity date extension | Commercial mortgages
Financing Receivable, Modifications [Line Items]
Loan Modification Total	1	2
Discharged in bankruptcy
Financing Receivable, Modifications [Line Items]
Loan Modification Total	17	3
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	5	1
Discharged in bankruptcy | Consumer
Financing Receivable, Modifications [Line Items]
Loan Modification Total	12	2
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
Other
Financing Receivable, Modifications [Line Items]
Loan Modification Total	10	8
Other | Commercial
Financing Receivable, Modifications [Line Items]
Loan Modification Total	0	1
Other | Construction
Financing Receivable, Modifications [Line Items]
Loan Modification Total	1	0
Other | Residential
Financing Receivable, Modifications [Line Items]
Loan Modification Total	1	7
Other | Consumer
Financing Receivable, Modifications [Line Items]
Loan Modification Total	8	0
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