Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	$ 5,169,476	$ 4,962,289
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	5,127,027	4,929,870
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	14,226	9,074
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	28,223	23,345
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Real estate loans | One-to-four family
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,844,507	1,830,186
Real estate loans | One-to-four family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,841,273	1,826,066
Real estate loans | One-to-four family | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Real estate loans | One-to-four family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	3,234	4,120
Real estate loans | One-to-four family | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Real estate loans | Multifamily and commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,316,922	2,142,154
Real estate loans | Multifamily and commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,303,084	2,128,680
Real estate loans | Multifamily and commercial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	140	0
Real estate loans | Multifamily and commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	13,698	13,474
Real estate loans | Multifamily and commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Real estate loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	268,522	261,473
Real estate loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	268,522	261,473
Real estate loans | Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Real estate loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Real estate loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Commercial business loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	375,412	333,876
Commercial business loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	351,083	320,451
Commercial business loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	14,086	9,074
Commercial business loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	10,243	4,351
Commercial business loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Consumer loans | Home equity loans and advances
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	362,432	393,492
Consumer loans | Home equity loans and advances | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	361,384	392,092
Consumer loans | Home equity loans and advances | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Consumer loans | Home equity loans and advances | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,048	1,400
Consumer loans | Home equity loans and advances | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Consumer loans | Other consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,681	1,108
Consumer loans | Other consumer loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,681	1,108
Consumer loans | Other consumer loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Consumer loans | Other consumer loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Consumer loans | Other consumer loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	$ 0	$ 0