Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for loan losses:
Beginning balance	$ 74,015	$ 62,403	$ 61,709	$ 62,342
Provision for loan losses	2,516	1,157	17,820	1,705
Recoveries	247	35	608	434
Charge-offs	(645)	(966)	(4,004)	(1,852)
Ending balance	76,133	62,629	76,133	62,629
Real estate loans | One-to-four family
Allowance for loan losses:
Beginning balance	16,633	15,610	13,780	15,232
Provision for loan losses	(758)	347	2,292	1,215
Recoveries	87	4	329	29
Charge-offs	(187)	(228)	(626)	(743)
Ending balance	15,775	15,733	15,775	15,733
Real estate loans | Multifamily and commercial
Allowance for loan losses:
Beginning balance	27,330	22,679	22,980	23,251
Provision for loan losses	3,659	290	7,998	(282)
Recoveries	0	0	12	0
Charge-offs	0	0	(1)	0
Ending balance	30,989	22,969	30,989	22,969
Real estate loans | Construction
Allowance for loan losses:
Beginning balance	10,217	8,806	7,435	7,217
Provision for loan losses	317	(1,047)	3,098	541
Recoveries	0	1	1	2
Charge-offs	0	0	0	0
Ending balance	10,534	7,760	10,534	7,760
Commercial business loans
Allowance for loan losses:
Beginning balance	18,314	12,718	15,836	14,176
Provision for loan losses	(573)	2,040	4,685	485
Recoveries	128	8	211	374
Charge-offs	(367)	(738)	(3,230)	(1,007)
Ending balance	17,502	14,028	17,502	14,028
Consumer loans | Home equity loans and advances
Allowance for loan losses:
Beginning balance	1,514	2,583	1,669	2,458
Provision for loan losses	(129)	(474)	(253)	(255)
Recoveries	32	22	55	29
Charge-offs	(91)	0	(145)	(101)
Ending balance	1,326	2,131	1,326	2,131
Consumer loans | Other consumer loans
Allowance for loan losses:
Beginning balance	7	7	9	8
Provision for loan losses	0	1	0	1
Recoveries	0	0	0	0
Charge-offs	0	0	(2)	(1)
Ending balance	$ 7	$ 8	$ 7	$ 8