Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Narrative) (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) contracts	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2018 USD ($) contracts	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2018 USD ($) Loan	Sep. 30, 2017 USD ($) Loan	Dec. 31, 2017 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number	22	5	41	13	49	19
Allowance for loan losses related to impaired loans	$ 14,365,000	$ 7,218,000	$ 14,365,000	$ 7,218,000	$ 14,365,000	$ 7,218,000	$ 8,023,000
TDRs, Additions	$ 4,233,000	$ 1,663,000	$ 24,828,000	$ 5,386,000	43,767,000	8,834,000
Weighted average extension term	72 months	36 months	64 months	36 months
Weighted average stated interest rate, basis points	1.40%	2.25%	1.60%	1.88%
Loan forgiveness	$ 0	$ 73,000	$ 0	$ 73,000
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs	66,219,000	33,183,000	66,219,000	33,183,000	66,219,000	33,183,000	$ 49,786,000
Allowance for loan losses related to impaired loans	3,900,000		3,900,000		3,900,000
Interest income, passage of time	32,000	68,000	89,000	172,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Foreclosed residential properties	5,700,000		5,700,000		5,700,000
In process other real estate owned	$ 14,200,000	$ 12,100,000	$ 14,200,000	$ 12,100,000	$ 14,200,000	$ 12,100,000