Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	$ 4,994,129	$ 4,962,289
Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	4,956,541	4,929,870
Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	11,334	9,074
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	26,254	23,345
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	1,830,583	1,830,186
Real estate loans | One-to-four family | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	1,824,967	1,826,066
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	5,616	4,120
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	2,132,503	2,142,154
Real estate loans | Multifamily and commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	2,119,049	2,128,680
Real estate loans | Multifamily and commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	90	0
Real estate loans | Multifamily and commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	13,364	13,474
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	307,429	261,473
Real estate loans | Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	305,729	261,473
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	1,700	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	339,483	333,876
Commercial business loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	323,622	320,451
Commercial business loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	11,244	9,074
Commercial business loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	4,617	4,351
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	383,143	393,492
Consumer loans | Home equity loans and advances | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	382,186	392,092
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	957	1,400
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	988	1,108
Consumer loans | Other consumer loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total gross loans	988	1,108
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