Loans Receivable and Allowance for Loan Losses - Aging of Loans and Accrual Status (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 585	$ 1,020
Loans Not Past Due	1,541,238	1,586,476
Total ending loan balance	1,541,823	1,587,496
Nonaccrual Loans	763	736
Loans 90 Days or More Past Due and Still Accruing	29	1
One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	531	959
Loans Not Past Due	1,488,824	1,533,446
Total ending loan balance	1,489,355	1,534,405
Nonaccrual Loans	739	647
Multi-family residential
Financing Receivable, Recorded Investment, Past Due
Loans Not Past Due	8,971	9,944
Total ending loan balance	8,971	9,944
Construction, Commercial and Other Mortgage Loans
Financing Receivable, Recorded Investment, Past Due
Loans Not Past Due	22,109	23,326
Total ending loan balance	22,109	23,326
Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment, Past Due
Total Past Due	24	26
Loans Not Past Due	9,969	10,060
Total ending loan balance	9,993	10,086
Nonaccrual Loans	24	89
Loans on Deposit Accounts
Financing Receivable, Recorded Investment, Past Due
Loans Not Past Due	276	235
Total ending loan balance	276	235
Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due	30	35
Loans Not Past Due	11,089	9,465
Total ending loan balance	11,119	9,500
Loans 90 Days or More Past Due and Still Accruing	29	1
30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	233	33
30 - 59 Days Past Due | One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	232
30 - 59 Days Past Due | Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1	33
60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	188	986
60 - 89 Days Past Due | One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	164	959
60 - 89 Days Past Due | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment, Past Due
Total Past Due	24	26
60 - 89 Days Past Due | Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due		1
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	164	1
90 Days or More Past Due | One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	135
90 Days or More Past Due | Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 29	$ 1