Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	$ 4,962,289	$ 4,446,015
Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	4,929,870	4,405,719
Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	9,074	8,460
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	23,345	31,836
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Real estate loans | One-to-four family
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	1,830,186	1,615,000
Real estate loans | One-to-four family | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	1,826,066	1,605,413
Real estate loans | One-to-four family | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Real estate loans | One-to-four family | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	4,120	9,587
Real estate loans | One-to-four family | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Real estate loans | Multifamily and commercial
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	2,142,154	1,870,475
Real estate loans | Multifamily and commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	2,128,680	1,851,037
Real estate loans | Multifamily and commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	4,782
Real estate loans | Multifamily and commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	13,474	14,656
Real estate loans | Multifamily and commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Real estate loans | Construction
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	261,473	233,652
Real estate loans | Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	261,473	233,652
Real estate loans | Construction | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Real estate loans | Construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Real estate loans | Construction | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Commercial business loans
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	333,876	277,970
Commercial business loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	320,451	268,355
Commercial business loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	9,074	3,678
Commercial business loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	4,351	5,937
Commercial business loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Consumer loans | Home equity loans and advances
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	393,492	447,920
Consumer loans | Home equity loans and advances | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	392,092	446,264
Consumer loans | Home equity loans and advances | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Consumer loans | Home equity loans and advances | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	1,400	1,656
Consumer loans | Home equity loans and advances | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Consumer loans | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	1,108	998
Consumer loans | Other consumer loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	1,108	998
Consumer loans | Other consumer loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Consumer loans | Other consumer loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	0	0
Consumer loans | Other consumer loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	$ 0	$ 0