Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Narrative) (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) contracts	Mar. 31, 2017 USD ($) contracts	Mar. 31, 2018 USD ($) Loan	Mar. 31, 2017 USD ($) Loan	Dec. 31, 2017 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs, number	7	4	24	14
Allowance for loan losses related to impaired loans	$ 6,863,000	$ 8,165,000	$ 6,863,000	$ 8,165,000	$ 8,023,000
TDRs, Additions	$ 990,000	$ 1,513,000	23,802,000	5,687,000
Weighted average extension term	74 months	10 months
Weighted average stated interest rate, basis points	2.87%	0.48%
Loan forgiveness	$ 0	$ 0
Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
TDRs	$ 47,676,000	39,669,000	47,676,000	39,669,000	$ 49,786,000
TDRs, number | contracts	85
Allowance for loan losses related to impaired loans	$ 1,100,000		1,100,000
Interest income, passage of time	21,000	55,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Losses [Line Items]
Foreclosed residential properties	8,600,000		8,600,000
In process other real estate owned	$ 11,400,000	$ 13,500,000	$ 11,400,000	$ 13,500,000