Fair Value - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Provision expense for (recoveries from) credit losses	$ 0	$ 1,437	$ 39,495	$ 3,483
Net carrying amount other real estate owned and other repossessed property	106		106			$ 43
Other real estate owned property write-downs	63	18	161	39
Valuation allowance for loan servicing rights with impairments	2,348	121	2,348	121	$ 2,253	31	$ 32	$ 15
Valuation allowance for loan servicing rights with impairments, impairments during period	95	89	$ 2,317	106
Past due period of mortgage loans held for sale, days			90 days
Interest income for residential loans held for sale	600	400	$ 1,500	900
States and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3				4,000
Impaired Commercial and Commercial Real Estate Loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal amount of impaired commercial and commercial real estate loans	39,000		39,000			30,900
Valuation allowance	4,700		4,700			$ 8,900
Provision expense for (recoveries from) credit losses	$ (5,000)	$ 300	$ 1,500	$ 2,600