Summary of Significant Accounting Policies - Additional Information (Details)		12 Months Ended
	Mar. 27, 2018 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) Branch	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Jan. 01, 2019 USD ($)	Dec. 31, 2017 USD ($)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operations commenced date		Jun. 01, 2016
Number of full service branches | Branch		9
Interest and/or penalties related to income tax matters		$ 0	$ 0
Operating lease liabilities		8,429,000	10,126,000
Operating right-of-use assets		6,786,000	8,254,000
Cumulative-effect adjustment to retained earnings		$ 143,366,000	140,576,000	$ 129,787,000		$ 91,480,000
ASU 2016-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease liabilities					$ 9,900,000
Operating right-of-use assets			8,254,000		8,000,000.0
Equipment and Furnishings | Minimum
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Equipment and furnishings, useful lives		3 years
Equipment and Furnishings | Maximum
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Equipment and furnishings, useful lives		10 years
Common Stock
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Stock issued under stock offering, net of expenses, shares | shares				2,300,000
Proceeds from issuance initial public offering	$ 22,600,000
Cumulative-effect adjustment to retained earnings		$ 78,657,000	86,381,000	$ 91,209,000		67,926,000
Retained Earnings
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings		$ 55,348,000	$ 46,483,000	$ 32,877,000		$ 18,624,000
Retained Earnings | ASU 2016-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings					$ 0
Initial Public Offering | Common Stock
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Stock issued under stock offering, net of expenses, shares | shares	2,300,000
Stock price per share | $ / shares	$ 11.00
Payments for underwriting discounts and commissions	$ 1,700,000
Payments of stock offering expenses	$ 925,000
Open Bank
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Percentage of voting equity interests acquired		100.00%
Operations commenced date		Jun. 10, 2005