Loans Receivable and Allowance for Loan Losses - Aging of Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 18,745	$ 18,196
Current	5,077,512	4,944,093
Total	5,096,257	4,962,289
30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	9,270	12,012
60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	5,526	3,395
90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	3,949	2,789
Real estate loans | One-to-four family
Financing Receivable, Past Due [Line Items]
Total Past Due	11,465	10,721
Current	1,808,074	1,819,465
Total	1,819,539	1,830,186
Real estate loans | One-to-four family | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	8,246	8,384
Real estate loans | One-to-four family | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	799	1,518
Real estate loans | One-to-four family | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	2,420	819
Real estate loans | Multifamily and commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	1,847	3,449
Current	2,246,453	2,138,705
Total	2,248,300	2,142,154
Real estate loans | Multifamily and commercial | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	1,870
Real estate loans | Multifamily and commercial | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	1,396	1,425
Real estate loans | Multifamily and commercial | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	451	154
Real estate loans | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	1,700	0
Current	291,850	261,473
Total	293,550	261,473
Real estate loans | Construction | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Real estate loans | Construction | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	1,700	0
Real estate loans | Construction | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Commercial business loans
Financing Receivable, Past Due [Line Items]
Total Past Due	1,349	1,398
Current	357,271	332,478
Total	358,620	333,876
Commercial business loans | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	126	208
Commercial business loans | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	534	279
Commercial business loans | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	689	911
Consumer loans | Home equity loans and advances
Financing Receivable, Past Due [Line Items]
Total Past Due	2,384	2,628
Current	372,740	390,864
Total	375,124	393,492
Consumer loans | Home equity loans and advances | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	898	1,550
Consumer loans | Home equity loans and advances | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	1,097	173
Consumer loans | Home equity loans and advances | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	389	905
Consumer loans | Other consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	1,124	1,108
Total	1,124	1,108
Consumer loans | Other consumer loans | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Consumer loans | Other consumer loans | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Consumer loans | Other consumer loans | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 0	$ 0