Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	$ 6,176,329	$ 4,962,289
Real estate loans | One-to-four family
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,077,508	1,830,186
Real estate loans | Multifamily and commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,924,851	2,142,154
Real estate loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	298,942	261,473
Commercial business loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	484,941	333,876
Consumer loans | Home equity loans and advances
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	388,127	393,492
Consumer loans | Other consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,960	1,108
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	6,169,308	4,962,289
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	6,115,500	4,929,870
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	25,313	9,074
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	28,495	23,345
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,077,079	1,830,186
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,072,878	1,826,066
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	419	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	3,782	4,120
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,919,985	2,142,154
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,900,286	2,128,680
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	4,724	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	14,975	13,474
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	298,942	261,473
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	298,942	261,473
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	483,215	333,876
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	454,183	320,451
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	20,170	9,074
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	8,862	4,351
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	388,127	393,492
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	387,251	392,092
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	876	1,400
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,960	1,108
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,960	1,108
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	$ 0	$ 0