Loans Receivable - Troubled Debt Restructurings (Details) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) contract	Dec. 31, 2018 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total TDR loans	$ 3,544	$ 3,745
Allowance for loan losses related to TDR loans	41	43
Total nonaccrual TDR loans	$ 81	$ 84
Pre-modification, number of contracts \| contract	1	3
Pre-modification outstanding recorded investment	$ 50	$ 229
Post-modification outstanding recorded investment, number of contracts \| contract	1	3
Post-modification outstanding recorded investment	$ 51	$ 228
Residential segment \| Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total TDR loans	643	663
Total nonaccrual TDR loans	0	0
Consumer \| Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total TDR loans	160	258
Total nonaccrual TDR loans	0	0
Rate Modification
Financing Receivable, Troubled Debt Restructuring [Line Items]
Pre-modification outstanding recorded investment	0	0
Post-modification outstanding recorded investment	0	0
Term Modification
Financing Receivable, Troubled Debt Restructuring [Line Items]
Pre-modification outstanding recorded investment	0	0
Post-modification outstanding recorded investment	0	0
Combination Modification
Financing Receivable, Troubled Debt Restructuring [Line Items]
Pre-modification outstanding recorded investment	50	229
Post-modification outstanding recorded investment	51	228
One- to four-family \| Residential segment \| Real estate loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total TDR loans	2,371	2,442
Total nonaccrual TDR loans	$ 81	$ 84
Pre-modification, number of contracts \| contract	1	3
Pre-modification outstanding recorded investment	$ 50	$ 229
Post-modification outstanding recorded investment, number of contracts \| contract	1	3
Post-modification outstanding recorded investment	$ 51	$ 228
Modifications that subsequently defaulted, number of contracts \| contract	2	2
TDR loans that subsequently defaulted	$ 99	$ 140
One- to four-family \| Rate Modification \| Residential segment \| Real estate loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Pre-modification outstanding recorded investment	0	0
Post-modification outstanding recorded investment	0	0
TDR loans that subsequently defaulted	0	0
One- to four-family \| Term Modification \| Residential segment \| Real estate loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Pre-modification outstanding recorded investment	0	0
Post-modification outstanding recorded investment	0	0
TDR loans that subsequently defaulted	0	0
One- to four-family \| Combination Modification \| Residential segment \| Real estate loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Pre-modification outstanding recorded investment	50	229
Post-modification outstanding recorded investment	51	228
TDR loans that subsequently defaulted	99	140
Multi-family \| Residential segment \| Real estate loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total TDR loans	107	110
Total nonaccrual TDR loans	$ 0	$ 0

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