Loans Receivable and Allowance for Loan Losses - Aging of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 12,978	$ 18,196
Current	5,156,498	4,944,093
Total	5,169,476	4,962,289
30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	6,505	12,012
60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	2,609	3,395
90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	3,864	2,789
Real estate loans | One-to-four family
Financing Receivable, Past Due [Line Items]
Total Past Due	8,931	10,721
Current	1,835,576	1,819,465
Total	1,844,507	1,830,186
Real estate loans | One-to-four family | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	5,326	8,384
Real estate loans | One-to-four family | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	1,958	1,518
Real estate loans | One-to-four family | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	1,647	819
Real estate loans | Multifamily and commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	1,156	3,449
Current	2,315,766	2,138,705
Total	2,316,922	2,142,154
Real estate loans | Multifamily and commercial | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	439	1,870
Real estate loans | Multifamily and commercial | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	114	1,425
Real estate loans | Multifamily and commercial | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	603	154
Real estate loans | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	268,522	261,473
Total	268,522	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	0	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	1,129	1,398
Current	374,283	332,478
Total	375,412	333,876
Commercial business loans | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	208
Commercial business loans | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0	279
Commercial business loans | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	1,129	911
Consumer loans | Home equity loans and advances
Financing Receivable, Past Due [Line Items]
Total Past Due	1,762	2,628
Current	360,670	390,864
Total	362,432	393,492
Consumer loans | Home equity loans and advances | 30-59 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	740	1,550
Consumer loans | Home equity loans and advances | 60-89 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	537	173
Consumer loans | Home equity loans and advances | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	485	905
Consumer loans | Other consumer loans
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	1,681	1,108
Total	1,681	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