Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	$ 842		$ 546
Allowance for loan losses, Collectively evaluated for impairment	61,682		57,632
Total	62,524	$ 59,952	58,178	$ 52,066	$ 51,908	$ 51,849
Total loans:
Total loans, Individually evaluated for impairment	19,848		22,191
Total loans, Collectively evaluated for impairment	4,677,420		4,425,918
Total	4,697,268		4,448,109
Real estate loans | One-to-four family
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	476		423
Allowance for loan losses, Collectively evaluated for impairment	18,073		19,568
Total	18,549	18,828	19,991	18,762	18,818	18,599
Total loans:
Total loans, Individually evaluated for impairment	10,605		11,644
Total loans, Collectively evaluated for impairment	1,752,553		1,604,615
Total	1,763,158		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	22,802		19,905
Total	22,802	19,097	19,933	18,085	17,638	17,616
Total loans:
Total loans, Individually evaluated for impairment	2,856		3,693
Total loans, Collectively evaluated for impairment	1,968,947		1,867,517
Total	1,971,803		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	6,728		5,217
Total	6,728	6,574	5,217	4,598	4,253	4,598
Total loans:
Total loans, Individually evaluated for impairment	0		0
Total loans, Collectively evaluated for impairment	254,850		233,652
Total	254,850		233,652
Commercial business loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	355		80
Allowance for loan losses, Collectively evaluated for impairment	10,565		8,195
Total	10,920	10,800	8,275	6,563	7,014	6,358
Total loans:
Total loans, Individually evaluated for impairment	2,951		4,263
Total loans, Collectively evaluated for impairment	289,162		273,707
Total	292,113		277,970
Consumer loans | Home equity loans and advances
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	11		15
Allowance for loan losses, Collectively evaluated for impairment	2,859		4,562
Total	2,870	3,924	4,577	3,965	4,129	4,231
Total loans:
Total loans, Individually evaluated for impairment	3,436		2,591
Total loans, Collectively evaluated for impairment	410,952		445,429
Total	414,388		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	10	11
Total loans:
Total loans, Individually evaluated for impairment	0		0
Total loans, Collectively evaluated for impairment	956		998
Total	956		998
Unallocated
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0		0
Allowance for loan losses, Collectively evaluated for impairment	648		177
Total	648	$ 722	177	$ 85	$ 46	$ 436
Total loans:
Total loans, Individually evaluated for impairment	0		0
Total loans, Collectively evaluated for impairment	0		0
Total	$ 0		$ 0