Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	$ 884		$ 546
Allowance for loan losses, Collectively evaluated for impairment	62,522		57,632
Total	63,406	$ 62,524	58,178	$ 54,633	$ 52,066	$ 51,849
Total loans:
Total loans, Individually evaluated for impairment	19,891		22,191
Total loans, Collectively evaluated for impairment	4,871,511		4,425,918
Total	4,891,402		4,448,109
Real estate loans | One-to-four family
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	473		423
Allowance for loan losses, Collectively evaluated for impairment	17,052		19,568
Total	17,525	18,549	19,991	18,533	18,762	18,599
Total loans:
Total loans, Individually evaluated for impairment	10,416		11,644
Total loans, Collectively evaluated for impairment	1,812,850		1,604,615
Total	1,823,266		1,616,259
Real estate loans | Multifamily and commercial
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0		28
Allowance for loan losses, Collectively evaluated for impairment	23,767		19,905
Total	23,767	22,802	19,933	18,029	18,085	17,616
Total loans:
Total loans, Individually evaluated for impairment	2,717		3,693
Total loans, Collectively evaluated for impairment	2,086,413		1,867,517
Total	2,089,130		1,871,210
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	7,148		5,217
Total	7,148	6,728	5,217	5,299	4,598	4,598
Total loans:
Total loans, Individually evaluated for impairment	0		0
Total loans, Collectively evaluated for impairment	269,729		233,652
Total	269,729		233,652
Commercial business loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	377		80
Allowance for loan losses, Collectively evaluated for impairment	11,488		8,195
Total	11,865	10,920	8,275	8,480	6,563	6,358
Total loans:
Total loans, Individually evaluated for impairment	2,936		4,263
Total loans, Collectively evaluated for impairment	301,285		273,707
Total	304,221		277,970
Consumer loans | Home equity loans and advances
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	34		15
Allowance for loan losses, Collectively evaluated for impairment	2,809		4,562
Total	2,843	2,870	4,577	4,190	3,965	4,231
Total loans:
Total loans, Individually evaluated for impairment	3,822		2,591
Total loans, Collectively evaluated for impairment	400,206		445,429
Total	404,028		448,020
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	7	8	8	8	11
Total loans:
Total loans, Individually evaluated for impairment	0		0
Total loans, Collectively evaluated for impairment	1,028		998
Total	1,028		998
Unallocated
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0		0
Allowance for loan losses, Collectively evaluated for impairment	251		177
Total	251	$ 648	177	$ 94	$ 85	$ 436
Total loans:
Total loans, Individually evaluated for impairment	0		0
Total loans, Collectively evaluated for impairment	0		0
Total	$ 0		$ 0