Investment Securities (ASU 2016-13 Adoption Impact) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, debt securities, allowance	$ 42	$ 78		$ 0
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, debt securities, allowance			$ 70
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, debt securities, allowance			70
Corporate obligations
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, debt securities, allowance				0
Corporate obligations | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, debt securities, allowance			6
Corporate obligations | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, debt securities, allowance			6
Municipal securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, debt securities, allowance				$ 0
Municipal securities | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, debt securities, allowance			64
Municipal securities | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, debt securities, allowance			$ 64