Loans Receivable and Allowance for Loan Losses - Non-accrual Status of Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	$ 6,694	$ 2,789
Real estate loans | One-to-four family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	2,567	819
Real estate loans | Multifamily and commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	451	154
Real estate loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	1,700	0
Commercial business loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	1,384	911
Consumer loans | Home equity loans and advances
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	$ 592	$ 905