Fair Value - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net carrying amount other real estate owned and other repossessed property	$ 0		$ 0			$ 68,000
Other real estate owned property write-downs	18,000	$ 100,000	39,000	$ 500,000
Valuation allowance for loan servicing rights with impairments	121,000	12,000	121,000	12,000	$ 32,000	15,000	$ 15,000	$ 29,000
Recoveries on loan servicing rights	89,000	(3,000)	$ 106,000	(17,000)
Past due period of mortgage loans held for sale, days			90 days
Interest income for residential loans held for sale	400,000	200,000	$ 900,000	300,000
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3			4,000,000.0
Transfers into Level 3				4,100,000
Impaired Commercial and Commercial Real Estate Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal amount of impaired commercial and commercial real estate loans	39,700,000		39,700,000			49,300,000
Valuation allowance	11,200,000		11,200,000			$ 12,900,000
Provision for loan losses expensed	$ 300,000	$ 1,200,000	$ 2,600,000	$ 7,700,000