Loans Receivable and Allowance for Loan Losses - Aging of Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 13,866	$ 18,196
Current	4,980,263	4,944,093
Total	4,994,129	4,962,289
30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,225	12,012
60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,232	3,395
90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,409	2,789
Real estate loans | One-to-four family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	9,840	10,721
Current	1,820,743	1,819,465
Total	1,830,583	1,830,186
Real estate loans | One-to-four family | 30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,181	8,384
Real estate loans | One-to-four family | 60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,533	1,518
Real estate loans | One-to-four family | 90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,126	819
Real estate loans | Multifamily and commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	967	3,449
Current	2,131,536	2,138,705
Total	2,132,503	2,142,154
Real estate loans | Multifamily and commercial | 30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	1,870
Real estate loans | Multifamily and commercial | 60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	813	1,425
Real estate loans | Multifamily and commercial | 90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	154	154
Real estate loans | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Current	307,429	261,473
Total	307,429	261,473
Real estate loans | Construction | 30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Real estate loans | Construction | 60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Real estate loans | Construction | 90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Commercial business loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	766	1,398
Current	338,717	332,478
Total	339,483	333,876
Commercial business loans | 30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	77	208
Commercial business loans | 60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	279
Commercial business loans | 90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	689	911
Consumer loans | Home equity loans and advances
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,293	2,628
Current	380,850	390,864
Total	383,143	393,492
Consumer loans | Home equity loans and advances | 30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	967	1,550
Consumer loans | Home equity loans and advances | 60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	886	173
Consumer loans | Home equity loans and advances | 90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	440	905
Consumer loans | Other consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Current	988	1,108
Total	988	1,108
Consumer loans | Other consumer loans | 30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Consumer loans | Other consumer loans | 60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Consumer loans | Other consumer loans | 90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 0	$ 0