Loans Receivable and Allowance for Loan and Lease Losses Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2019
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, Recorded Investment	$ 9,549		$ 9,549		$ 12,369
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, Unpaid Principal Balance	10,612		10,612		13,567
Impaired Financing Receivable, Related Allowance	182		182		189
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, Average Recorded Investment	10,609	$ 12,276	10,959	$ 12,598
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, Interest Income, Cash Basis Method	44	54	112	68
One-to-four family
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	3,994		3,994		4,488
Impaired Financing Receivable, with Related Allowance, Recorded Investment	815		815		929
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	4,999		4,999		5,643
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	815		815		929
Impaired Financing Receivable, Related Allowance	169		169		171
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	4,185	4,813	4,241	5,074
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	869	887	872	1,003
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	14	26	30	31
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	1	0	2
Multifamily
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,433		2,433		3,214
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,433		2,433		3,214
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,789	2,382	2,824	1,646
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0	0	0	371
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	14	9	41	17
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0	0	0
Commercial real estate
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0		0		476
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	0		0		476
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	0	492	238	1,014
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	8	0	8
Business
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,418		1,418		1,974
Impaired Financing Receivable, with Related Allowance, Recorded Investment	889		889		1,288
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	1,476		1,476		2,017
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	889		889		1,288
Impaired Financing Receivable, Related Allowance	13		13		$ 18
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	1,699	652	1,696	623
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,067	3,050	1,088	2,867
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	16	6	41	6
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	$ 0	$ 4	$ 0	$ 4