Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Narrative) (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) contracts	Jun. 30, 2017 USD ($) contracts	Jun. 30, 2018 USD ($) contracts credits	Jun. 30, 2017 USD ($) contracts	Jun. 30, 2018 USD ($) Loan	Jun. 30, 2017 USD ($) Loan	Dec. 31, 2017 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number	12	4	19	8	32	16
Allowance for loan losses related to impaired loans	$ 8,541,000	$ 5,633,000	$ 8,541,000	$ 5,633,000	$ 8,541,000	$ 5,633,000	$ 8,023,000
TDRs, Additions	$ 19,605,000	$ 2,210,000	$ 20,595,000	$ 3,723,000	41,197,000	7,242,000
Weighted average extension term	37 months	54 months	50 months	36 months
Weighted average stated interest rate, basis points	1.57%	1.95%	1.73%	1.84%
Loan forgiveness	$ 0	$ 0	$ 0	$ 0
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs	65,310,000	33,091,000	$ 65,310,000	33,091,000	65,310,000	33,091,000	$ 49,786,000
TDRs, number | credits			94
Allowance for loan losses related to impaired loans	1,100,000		$ 1,100,000		1,100,000
Interest income, passage of time	36,000	49,000	57,000	104,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Foreclosed residential properties	7,400,000		7,400,000		7,400,000
In process other real estate owned	$ 11,200,000	$ 12,400,000	$ 11,200,000	$ 12,400,000	$ 11,200,000	$ 12,400,000