Loans Receivable and Allowance for Loan Losses - Aging of Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Non-accrual	$ 13,500	$ 6,700
Total	6,632,347	6,176,329
Real estate loans | One-to-four family
Financing Receivable, Past Due [Line Items]
Total	2,164,482	2,077,508
Real estate loans | Multifamily and commercial
Financing Receivable, Past Due [Line Items]
Total	2,878,965	2,924,851
Real estate loans | Construction
Financing Receivable, Past Due [Line Items]
Total	326,343	298,942
Commercial business loans
Financing Receivable, Past Due [Line Items]
Total	901,144	484,941
Consumer loans | Home equity loans and advances
Financing Receivable, Past Due [Line Items]
Total	359,813	388,127
Consumer loans | Other consumer loans
Financing Receivable, Past Due [Line Items]
Total	1,600	1,960
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration
Financing Receivable, Past Due [Line Items]
Total Past Due	28,725	18,478
Non-accrual	13,502	6,687
Current	6,583,239	6,144,143
Total	6,625,466	6,169,308
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	11,453	9,642
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	8,000	3,588
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	9,272	5,248
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	8,503	10,019
Non-accrual	4,872	1,732
Current	2,150,810	2,065,328
Total	2,164,185	2,077,079
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	4,105	6,249
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	1,120	2,132
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	3,278	1,638
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	8,071	2,552
Non-accrual	2,368	716
Current	2,863,580	2,916,717
Total	2,874,019	2,919,985
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	3,830	626
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	2,072	1,210
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	2,169	716
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	1,580	0
Non-accrual	0	0
Current	324,763	298,942
Total	326,343	298,942
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	1,580	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	8,569	3,545
Non-accrual	5,167	3,686
Current	885,770	475,984
Total	899,506	483,215
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	594	1,056
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	4,460	0
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	3,515	2,489
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	2,002	2,359
Non-accrual	1,095	553
Current	356,716	385,215
Total	359,813	388,127
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	1,344	1,708
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	348	246
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	310	405
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	0	3
Non-accrual	0	0
Current	1,600	1,957
Total	1,600	1,960
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	0	3
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