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, 2019 USD ($) contracts	Jun. 30, 2018 USD ($) contracts	Jun. 30, 2019 USD ($) contracts	Jun. 30, 2018 USD ($) contracts	Jun. 30, 2019 USD ($) Loan	Jun. 30, 2018 USD ($) Loan	Dec. 31, 2018 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
TDRs, number	30	12	60	19	112	32
Allowance for loan losses related to impaired loans	$ 15,200,000	$ 8,541,000	$ 15,200,000	$ 8,541,000	$ 15,200,000	$ 8,541,000	$ 11,437,000
TDRs, Additions	$ 9,489,000	$ 19,605,000	$ 33,207,000	$ 20,595,000	41,854,000	41,197,000
Weighted average extension term	15 months	37 months	14 years	50 years
Weighted average decrease in stated interest rate	2.19%	1.57%	2.16%	1.73%
Interest-only payment terms	6 months	7 months	3 months	7 months
Loan forgiveness	$ 0	$ 0	$ 0	$ 0
Financing Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
TDRs	76,001,000	65,310,000	$ 76,001,000	65,310,000	76,001,000	65,310,000	$ 66,102,000
TDRs, number | contracts			182
Allowance for loan losses related to impaired loans	3,800,000		$ 3,800,000		3,800,000
Interest income, passage of time	32,000	36,000	66,000	57,000
Residential Real Estate | Financing Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Foreclosed residential properties	2,600,000		2,600,000		2,600,000
In process other real estate owned	$ 10,400,000	$ 11,200,000	$ 10,400,000	$ 11,200,000	$ 10,400,000	$ 11,200,000