Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	$ 1,342	$ 915
Allowance for loan losses, Collectively evaluated for impairment	61,429	61,427
Total	62,771	62,342	$ 59,952	$ 58,178
Total loans:
Total loans, Individually evaluated for impairment	23,404	17,787
Total loans, Collectively evaluated for impairment	4,970,725	4,944,502
Total	4,994,129	4,962,289
Real estate loans | One-to-four family
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	534	537
Allowance for loan losses, Collectively evaluated for impairment	16,841	14,695
Total	17,375	15,232	18,828	19,991
Total loans:
Total loans, Individually evaluated for impairment	8,566	9,048
Total loans, Collectively evaluated for impairment	1,822,017	1,821,138
Total	1,830,583	1,830,186
Real estate loans | Multifamily and commercial
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	1	0
Allowance for loan losses, Collectively evaluated for impairment	20,985	23,251
Total	20,986	23,251	19,097	19,933
Total loans:
Total loans, Individually evaluated for impairment	2,671	2,695
Total loans, Collectively evaluated for impairment	2,129,832	2,139,459
Total	2,132,503	2,142,154
Real estate loans | Construction
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	381	0
Allowance for loan losses, Collectively evaluated for impairment	8,652	7,217
Total	9,033	7,217	6,574	5,217
Total loans:
Total loans, Individually evaluated for impairment	1,700	0
Total loans, Collectively evaluated for impairment	305,729	261,473
Total	307,429	261,473
Commercial business loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	410	366
Allowance for loan losses, Collectively evaluated for impairment	11,815	13,810
Total	12,225	14,176	10,800	8,275
Total loans:
Total loans, Individually evaluated for impairment	7,874	2,944
Total loans, Collectively evaluated for impairment	331,609	330,932
Total	339,483	333,876
Consumer loans | Home equity loans and advances
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	16	12
Allowance for loan losses, Collectively evaluated for impairment	3,130	2,446
Total	3,146	2,458	3,924	4,576
Total loans:
Total loans, Individually evaluated for impairment	2,593	3,100
Total loans, Collectively evaluated for impairment	380,550	390,392
Total	383,143	393,492
Consumer loans | Other consumer loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0	0
Allowance for loan losses, Collectively evaluated for impairment	6	8
Total	6	8	7	8
Total loans:
Total loans, Individually evaluated for impairment	0	0
Total loans, Collectively evaluated for impairment	988	1,108
Total	988	1,108
Unallocated
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0	0
Allowance for loan losses, Collectively evaluated for impairment	0	0
Total	0	0	$ 722	$ 178
Total loans:
Total loans, Individually evaluated for impairment	0	0
Total loans, Collectively evaluated for impairment	0	0
Total	$ 0	$ 0