Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 4,891,402	$ 4,448,109
Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,854,149	4,407,813
Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,214	8,460
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	26,039	31,836
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | One-to-four family
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,823,266	1,616,259
Real estate loans | One-to-four family | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,816,636	1,606,672
Real estate loans | One-to-four family | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | One-to-four family | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,630	9,587
Real estate loans | One-to-four family | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | Multifamily and commercial
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,089,130	1,871,210
Real estate loans | Multifamily and commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,074,363	1,851,772
Real estate loans | Multifamily and commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,340	4,782
Real estate loans | Multifamily and commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	13,427	14,656
Real estate loans | Multifamily and commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | Construction
Financing Receivable, Recorded Investment [Line Items]
Total loans	269,729	233,652
Real estate loans | Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	269,729	233,652
Real estate loans | Construction | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | Construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | Construction | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial business loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	304,221	277,970
Commercial business loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	290,382	268,355
Commercial business loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	9,874	3,678
Commercial business loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,965	5,937
Commercial business loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Home equity loans and advances
Financing Receivable, Recorded Investment [Line Items]
Total loans	404,028	448,020
Consumer loans | Home equity loans and advances | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	402,011	446,364
Consumer loans | Home equity loans and advances | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Home equity loans and advances | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,017	1,656
Consumer loans | Home equity loans and advances | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,028	998
Consumer loans | Other consumer loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,028	998
Consumer loans | Other consumer loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Other consumer loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Other consumer loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0	$ 0