Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	$ 1,309		$ 915
Allowance for loan losses, Collectively evaluated for impairment	61,094		61,427
Total	62,403	$ 62,771	62,342	$ 62,524	$ 59,952	$ 58,178
Total loans:
Total loans, individually evaluated for impairment	23,134		17,787
Total loans, collectively evaluated for impairment	5,073,123		4,944,502
Total	5,096,257		4,962,289
Real estate loans | One-to-four family
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	520		537
Allowance for loan losses, Collectively evaluated for impairment	15,090		14,695
Total	15,610	17,375	15,232	18,549	18,828	19,991
Total loans:
Total loans, individually evaluated for impairment	8,414		9,048
Total loans, collectively evaluated for impairment	1,811,125		1,821,138
Total	1,819,539		1,830,186
Real estate loans | Multifamily and commercial
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	2		0
Allowance for loan losses, Collectively evaluated for impairment	22,677		23,251
Total	22,679	20,986	23,251	22,802	19,097	19,933
Total loans:
Total loans, individually evaluated for impairment	2,648		2,695
Total loans, collectively evaluated for impairment	2,245,652		2,139,459
Total	2,248,300		2,142,154
Real estate loans | Construction
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	372		0
Allowance for loan losses, Collectively evaluated for impairment	8,434		7,217
Total	8,806	9,033	7,217	6,728	6,574	5,217
Total loans:
Total loans, individually evaluated for impairment	1,700		0
Total loans, collectively evaluated for impairment	291,850		261,473
Total	293,550		261,473
Commercial business loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	404		366
Allowance for loan losses, Collectively evaluated for impairment	12,314		13,810
Total	12,718	12,225	14,176	10,920	10,800	8,275
Total loans:
Total loans, individually evaluated for impairment	7,822		2,944
Total loans, collectively evaluated for impairment	350,798		330,932
Total	358,620		333,876
Consumer loans | Home equity loans and advances
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	11		12
Allowance for loan losses, Collectively evaluated for impairment	2,572		2,446
Total	2,583	3,146	2,458	2,870	3,924	4,577
Total loans:
Total loans, individually evaluated for impairment	2,550		3,100
Total loans, collectively evaluated for impairment	372,574		390,392
Total	375,124		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	7		8
Total	7	6	8	7	7	8
Total loans:
Total loans, individually evaluated for impairment	0		0
Total loans, collectively evaluated for impairment	1,124		1,108
Total	1,124		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	0	$ 648	$ 722	$ 177
Total loans:
Total loans, individually evaluated for impairment	0		0
Total loans, collectively evaluated for impairment	0		0
Total	$ 0		$ 0