Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	$ 6,458,807	$ 6,176,329
Real estate loans | One-to-four family
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,053,584	2,077,508
Real estate loans | Multifamily and commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,868,641	2,924,851
Real estate loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	307,659	298,942
Commercial business loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	886,423	484,941
Consumer loans | Home equity loans and advances
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	340,962	388,127
Consumer loans | Other consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,538	1,960
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	6,452,101	6,169,308
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	6,393,000	6,115,500
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	24,288	25,313
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	34,813	28,495
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 | Loss
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	2,053,293	2,077,079
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	2,046,613	2,072,878
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	408	419
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	6,272	3,782
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 | One-to-four family | Loss
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	2,863,718	2,919,985
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	2,828,021	2,900,286
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	15,819	4,724
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	19,878	14,975
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 | Multifamily and commercial | Loss
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	307,659	298,942
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	307,659	298,942
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 | Real estate loans | Construction | Loss
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	884,931	483,215
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	869,122	454,183
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	8,061	20,170
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Commercial business loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	7,748	8,862
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 | Commercial business loans | Loss
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	340,962	388,127
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	340,047	387,251
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	915	876
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 | Home equity loans and advances | Loss
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	1,538	1,960
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	1,538	1,960
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
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Consumer loans | Other consumer loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	$ 0	$ 0