Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	$ 6,632,347	$ 6,176,329
Real estate loans | One-to-four family
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,164,482	2,077,508
Real estate loans | Multifamily and commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	2,878,965	2,924,851
Real estate loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	326,343	298,942
Commercial business loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	901,144	484,941
Consumer loans | Home equity loans and advances
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	359,813	388,127
Consumer loans | Other consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	1,600	1,960
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	6,625,466	6,169,308
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	6,571,826	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	18,020	25,313
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total gross loans	35,620	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	2,164,185	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	2,157,351	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	411	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	6,423	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	2,874,019	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	2,850,202	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	4,507	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	19,310	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	326,343	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	326,343	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	899,506	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	877,804	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	13,102	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	8,600	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	359,813	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	358,526	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	1,287	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	1,600	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	1,600	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