Loans Receivable, Net - Nonaccrual loans (Details) - Non-performing - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	$ 6,776	$ 10,294
One-to-four family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	3,582	4,488
Multifamily
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	375	3,214
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	0	476
Business
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	2,797	2,051
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	$ 22	$ 65