Loans Receivable and Allowance for Loan Losses - Aging of Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total	$ 6,176,329	$ 4,962,289
Real estate loans | One-to-four family
Financing Receivable, Past Due [Line Items]
Total	2,077,508	1,830,186
Real estate loans | Multifamily and commercial
Financing Receivable, Past Due [Line Items]
Total	2,924,851	2,142,154
Real estate loans | Construction
Financing Receivable, Past Due [Line Items]
Total	298,942	261,473
Commercial business loans
Financing Receivable, Past Due [Line Items]
Total	484,941	333,876
Consumer loans | Home equity loans and advances
Financing Receivable, Past Due [Line Items]
Total	388,127	393,492
Consumer loans | Other consumer loans
Financing Receivable, Past Due [Line Items]
Total	1,960	1,108
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration
Financing Receivable, Past Due [Line Items]
Total Past Due	18,478	18,196
Current	6,150,830	4,944,093
Total	6,169,308	4,962,289
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	9,642	12,012
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	3,588	3,395
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	5,248	2,789
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family
Financing Receivable, Past Due [Line Items]
Total Past Due	10,019	10,721
Current	2,067,060	1,819,465
Total	2,077,079	1,830,186
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	6,249	8,384
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	2,132	1,518
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	1,638	819
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	2,552	3,449
Current	2,917,433	2,138,705
Total	2,919,985	2,142,154
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	626	1,870
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	1,210	1,425
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	716	154
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	298,942	261,473
Total	298,942	261,473
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans
Financing Receivable, Past Due [Line Items]
Total Past Due	3,545	1,398
Current	479,670	332,478
Total	483,215	333,876
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	1,056	208
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	279
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	2,489	911
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances
Financing Receivable, Past Due [Line Items]
Total Past Due	2,359	2,628
Current	385,768	390,864
Total	388,127	393,492
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	1,708	1,550
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	246	173
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	405	905
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	3	0
Current	1,957	1,108
Total	1,960	1,108
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	3	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 0	$ 0