Loans Receivable and Allowance for Loan Losses - Aging of Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 13,187	$ 17,618
Current	4,684,081	4,430,491
Total	4,697,268	4,448,109
30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	6,456	8,728
60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,970	2,365
Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,761	6,525
Real estate loans | One-to-four family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	9,625	11,669
Current	1,753,533	1,604,590
Total	1,763,158	1,616,259
Real estate loans | One-to-four family | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	5,484	7,080
Real estate loans | One-to-four family | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,923	1,229
Real estate loans | One-to-four family | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,218	3,360
Real estate loans | Multifamily and commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	282	1,847
Current	1,971,521	1,869,363
Total	1,971,803	1,871,210
Real estate loans | Multifamily and commercial | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	164	138
Real estate loans | Multifamily and commercial | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	380
Real estate loans | Multifamily and commercial | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	118	1,329
Real estate loans | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Current	254,850	233,652
Total	254,850	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 | Greater than 90 days
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	538	2,082
Current	291,575	275,888
Total	292,113	277,970
Commercial business loans | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	89
Commercial business loans | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	154	730
Commercial business loans | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	384	1,263
Consumer loans | Home equity loans and advances
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,741	2,020
Current	411,647	446,000
Total	414,388	448,020
Consumer loans | Home equity loans and advances | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	808	1,421
Consumer loans | Home equity loans and advances | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	892	26
Consumer loans | Home equity loans and advances | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,041	573
Consumer loans | Other consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1	0
Current	955	998
Total	956	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	1	0
Consumer loans | Other consumer loans | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 0	$ 0