Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 4,697,268	$ 4,448,109
Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,662,437	4,407,813
Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,874	8,460
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	28,957	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	1,763,158	1,616,259
Real estate loans | One-to-four family | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,754,543	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	8,615	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	1,971,803	1,871,210
Real estate loans | Multifamily and commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,955,847	1,851,772
Real estate loans | Multifamily and commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,402	4,782
Real estate loans | Multifamily and commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	13,554	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	254,850	233,652
Real estate loans | Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	254,850	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	292,113	277,970
Commercial business loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	283,807	268,355
Commercial business loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,472	3,678
Commercial business loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,834	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	414,388	448,020
Consumer loans | Home equity loans and advances | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	412,434	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	1,954	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	956	998
Consumer loans | Other consumer loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	956	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