Loans Receivable and Allowance for Loan Losses - Aging of Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 18,196	$ 17,618
Current	4,944,093	4,428,397
Total	4,962,289	4,446,015
30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	12,012	8,728
60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,395	2,365
90 Days or more
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,789	6,525
Real estate loans | One-to-four family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	10,721	11,669
Current	1,819,465	1,603,331
Total	1,830,186	1,615,000
Real estate loans | One-to-four family | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	8,384	7,080
Real estate loans | One-to-four family | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,518	1,229
Real estate loans | One-to-four family | 90 Days or more
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	819	3,360
Real estate loans | Multifamily and commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,449	1,847
Current	2,138,705	1,868,628
Total	2,142,154	1,870,475
Real estate loans | Multifamily and commercial | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,870	138
Real estate loans | Multifamily and commercial | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,425	380
Real estate loans | Multifamily and commercial | 90 Days or more
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	154	1,329
Real estate loans | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Current	261,473	233,652
Total	261,473	233,652
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	1,398	2,082
Current	332,478	275,888
Total	333,876	277,970
Commercial business loans | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	208	89
Commercial business loans | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	279	730
Commercial business loans | 90 Days or more
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	911	1,263
Consumer loans | Home equity loans and advances
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,628	2,020
Current	390,864	445,900
Total	393,492	447,920
Consumer loans | Home equity loans and advances | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,550	1,421
Consumer loans | Home equity loans and advances | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	173	26
Consumer loans | Home equity loans and advances | 90 Days or more
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	905	573
Consumer loans | Other consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Current	1,108	998
Total	1,108	998
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