Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	$ 1,130		$ 915
Allowance for loan losses, Collectively evaluated for impairment	61,499		61,427
Total	62,629	$ 62,403	62,342	$ 63,406	$ 62,524	$ 58,178
Total loans:
Total loans, individually evaluated for impairment	21,560		17,787
Total loans, collectively evaluated for impairment	5,147,916		4,944,502
Total	5,169,476		4,962,289
Real estate loans | One-to-four family
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	507		537
Allowance for loan losses, Collectively evaluated for impairment	15,226		14,695
Total	15,733	15,610	15,232	17,525	18,549	19,991
Total loans:
Total loans, individually evaluated for impairment	9,131		9,048
Total loans, collectively evaluated for impairment	1,835,376		1,821,138
Total	1,844,507		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	22,967		23,251
Total	22,969	22,679	23,251	23,767	22,802	19,933
Total loans:
Total loans, individually evaluated for impairment	2,624		2,695
Total loans, collectively evaluated for impairment	2,314,298		2,139,459
Total	2,316,922		2,142,154
Real estate loans | Construction
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0		0
Allowance for loan losses, Collectively evaluated for impairment	7,760		7,217
Total	7,760	8,806	7,217	7,148	6,728	5,217
Total loans:
Total loans, individually evaluated for impairment	0		0
Total loans, collectively evaluated for impairment	268,522		261,473
Total	268,522		261,473
Commercial business loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	607		366
Allowance for loan losses, Collectively evaluated for impairment	13,421		13,810
Total	14,028	12,718	14,176	11,865	10,920	8,275
Total loans:
Total loans, individually evaluated for impairment	7,099		2,944
Total loans, collectively evaluated for impairment	368,313		330,932
Total	375,412		333,876
Consumer loans | Home equity loans and advances
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	14		12
Allowance for loan losses, Collectively evaluated for impairment	2,117		2,446
Total	2,131	2,583	2,458	2,843	2,870	4,577
Total loans:
Total loans, individually evaluated for impairment	2,706		3,100
Total loans, collectively evaluated for impairment	359,726		390,392
Total	362,432		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	8		8
Total	8	7	8	7	7	8
Total loans:
Total loans, individually evaluated for impairment	0		0
Total loans, collectively evaluated for impairment	1,681		1,108
Total	1,681		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	$ 251	$ 648	$ 177
Total loans:
Total loans, individually evaluated for impairment	0		0
Total loans, collectively evaluated for impairment	0		0
Total	$ 0		$ 0