Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		3 Months Ended	9 Months Ended	12 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Increase to retained earnings for cumulative effect of adopting ASU		$ 846,934,000	$ 846,934,000	$ 815,488,000
Increase to retained earnings for cumulative effect of adopting ASU tax		925,000
Addition to allowance for credit losses for financial assets purchased with credit deterioration			502,000
Accrued interest receivable		28,700,000	$ 28,700,000	$ 27,400,000
Paycheck Protection Program, CARES Act [Member] | Small Business Administration (SBA) [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of processing fees			5.00%
Processing fees received from the SBA			$ 30,100,000
Cumulative Effect, Period of Adoption, Adjustment [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Increase to retained earnings for cumulative effect of adopting ASU	$ 2,300,000
ASU No. 2018-13 [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 01, 2020
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true
ASU No. 2016-13 [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 01, 2020
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	false
Addition to allowance for credit losses for financial assets purchased with credit deterioration	$ 1,200,000
Accrued interest receivable		$ 28,700,000	$ 28,700,000
Delinquency period placed on nonaccrual status loan			90 days
Commercial loans are charged off deemed uncollectible period		180 days	180 days
ASU No. 2016-13 [Member] | U.S. Treasury or U.S. Government-sponsored Entities and Agencies [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Percentage of available for sale securities held			95.00%	95.00%
ASU No. 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Increase to retained earnings for cumulative effect of adopting ASU	2,300,000
Increase to retained earnings for cumulative effect of adopting ASU tax	$ 925,000