Loans Receivable and Allowance for Loan Losses - Loans Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	$ 6,458,807	$ 6,176,329
Net deferred loan costs, fees and purchased premiums and discounts	10,038	21,237
Loans receivable	6,468,845	6,197,566
Real estate loans | One-to-four family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	2,053,584	2,077,508
Real estate loans | Multifamily and commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	2,868,641	2,924,851
Real estate loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	307,659	298,942
Commercial business loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	886,423	484,941
Consumer loans | Home equity loans and advances
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	340,962	388,127
Consumer loans | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	1,538	1,960
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	6,452,101	6,169,308
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	2,053,293	2,077,079
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	2,863,718	2,919,985
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	307,659	298,942
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	884,931	483,215
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	340,962	388,127
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	1,538	1,960
Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	6,706	7,021
Financial Asset Acquired with Credit Deterioration | Real estate loans | One-to-four family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	291	429
Financial Asset Acquired with Credit Deterioration | Real estate loans | Multifamily and commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	4,923	4,866
Financial Asset Acquired with Credit Deterioration | Real estate loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	0	0
Financial Asset Acquired with Credit Deterioration | Commercial business loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	1,492	1,726
Financial Asset Acquired with Credit Deterioration | Consumer loans | Home equity loans and advances
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	0	0
Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans	$ 0	$ 0