Loans Receivable and Allowance for Loan Losses - Aging of Loans and Accrual Status (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 1,020	$ 1,247
Loans Not Past Due	1,586,476	1,576,109
Total ending loan balance	1,587,496	1,577,356
Nonaccrual Loans	736	2,213
Loans 90 Days or More Past Due and Still Accruing	1
One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	959	1,170
Loans Not Past Due	1,533,446	1,526,949
Total ending loan balance	1,534,405	1,528,119
Nonaccrual Loans	647	2,065
Multi-family residential
Financing Receivable, Recorded Investment, Past Due
Loans Not Past Due	9,944	12,135
Total ending loan balance	9,944	12,135
Construction, Commercial and Other Mortgage Loans
Financing Receivable, Recorded Investment, Past Due
Loans Not Past Due	23,326	20,698
Total ending loan balance	23,326	20,698
Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment, Past Due
Total Past Due	26	70
Loans Not Past Due	10,060	11,023
Total ending loan balance	10,086	11,093
Nonaccrual Loans	89	148
Loans on Deposit Accounts
Financing Receivable, Recorded Investment, Past Due
Loans Not Past Due	235	357
Total ending loan balance	235	357
Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due	35	7
Loans Not Past Due	9,465	4,947
Total ending loan balance	9,500	4,954
Loans 90 Days or More Past Due and Still Accruing	1
30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	33	43
30 - 59 Days Past Due | One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due		40
30 - 59 Days Past Due | Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due	33	3
60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	986	325
60 - 89 Days Past Due | One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	959	292
60 - 89 Days Past Due | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment, Past Due
Total Past Due	26	29
60 - 89 Days Past Due | Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1	4
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1	879
90 Days or More Past Due | One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due		838
90 Days or More Past Due | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment, Past Due
Total Past Due		$ 41
90 Days or More Past Due | Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 1