Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		12 Months Ended
	Jan. 01, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Days from origination after which mortgage loan are sold		30 days
Loan upon which a default of principal or interest has existed for a period		90 days
Intangible assets, Estimated useful lives		7 years 2 months 12 days
Advertising costs		$ 3,300,000	$ 3,800,000	$ 3,500,000
Revenue recognition, description of performance obligation		the Company offers various services to customers that generate revenue. Contract performance typically occurs in one year or less.
Incremental cost of obtaining contract		true
Revenue recognition, description of payment terms		Revenue is recognized when the transactions occur or as services are performed over primarily monthly or quarterly periods. Payment is typically received in the period the transactions occur, or in some cases, within 90 days of the service period.
Increase to retained earnings for cumulative effect of adopting ASU		$ 871,161,000	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		$ 26,000,000.0	$ 24,500,000
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		$ 30,200,000
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
ASC 326 [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
Addition to allowance for credit losses for financial assets purchased with credit deterioration	$ 1,200,000
Accrued interest receivable		$ 26,000,000.0
Delinquency period placed on nonaccrual status loan		90 days
Commercial loans are charged off deemed uncollectible period		180 days
ASC 326 [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%
ASC 326 [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
Core Deposit Intangibles [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Intangible assets, Estimated useful lives		7 years 3 months 18 days
Core Deposit Intangibles [Member] | Minimum [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Intangible assets, Estimated useful lives		7 years
Core Deposit Intangibles [Member] | Maximum [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Intangible assets, Estimated useful lives		10 years
Customer Relationship Intangibles [Member] | Minimum [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Intangible assets, Estimated useful lives		3 years
Customer Relationship Intangibles [Member] | Maximum [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Intangible assets, Estimated useful lives		18 years