Loans Receivable and Allowance for Loan and Lease Losses Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, Recorded Investment	$ 12,156		$ 12,156		$ 13,037
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, Unpaid Principal Balance	13,731		13,731		14,621
Impaired Financing Receivable, Related Allowance	672		672		315
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, Average Recorded Investment	12,276	$ 14,574	12,598	$ 15,255
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, Interest Income, Cash Basis Method	54	12	68	46
One-to-four family
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	4,709		4,709		5,439
Impaired Financing Receivable, with Related Allowance, Recorded Investment	940		940		1,065
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	5,976		5,976		6,862
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	935		935		1,065
Impaired Financing Receivable, Related Allowance	172		172		145
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	4,813	5,428	5,074	5,313
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	887	1,081	1,003	1,085
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	26	6	31	12
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	1	0	2	0
Multifamily
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,328		2,328		964
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0		741
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,328		2,328		1,122
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0		741
Impaired Financing Receivable, Related Allowance	0		0		75
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,382	1,528	1,646	1,533
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0	0	371	0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	9	6	17	15
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method		0	0	0
Commercial real estate
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	489		489		1,539
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	489		489		1,539
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	492	1,923	1,014	2,469
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	8	0	8	19
Business
Impaired Financing Receivable, Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	635		635		611
Impaired Financing Receivable, with Related Allowance, Recorded Investment	3,055		3,055		2,678
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	940		940		611
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	3,063		3,063		2,681
Impaired Financing Receivable, Related Allowance	500		500		$ 95
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	652	2,037	623	2,262
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	3,050	2,577	2,867	2,593
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Impaired Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	6	0	6	0
Impaired Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	$ 4	$ 0	$ 4	$ 0