Loans Receivable and Allowance for Loan Losses - Non-accrual Status of Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, recorded investment, nonaccrual status	$ 6,687	$ 2,789
Commercial business loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, recorded investment, nonaccrual status	3,686	911
One-to-four family | Real estate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, recorded investment, nonaccrual status	1,732	819
Multifamily and commercial | Real estate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, recorded investment, nonaccrual status	716	154
Home equity loans and advances | Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, recorded investment, nonaccrual status	$ 553	$ 905