Loans Receivable and Allowance for Loan Losses - Aging of Loans and Accrual Status (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 2,195	$ 2,841
Loans Not Past Due	1,536,811	1,488,678
Total ending loan balance	1,539,006	1,491,519
Nonaccrual Loans	2,101	4,227
One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	2,151	2,796
Loans Not Past Due	1,485,214	1,438,725
Total ending loan balance	1,487,365	1,441,521
Nonaccrual Loans	1,945	4,062
Multi-family residential
Financing Receivable, Recorded Investment, Past Due
Loans Not Past Due	10,609	10,782
Total ending loan balance	10,609	10,782
Construction, Commercial and Other Mortgage Loans
Financing Receivable, Recorded Investment, Past Due
Loans Not Past Due	24,453	21,701
Total ending loan balance	24,453	21,701
Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment, Past Due
Total Past Due	41	41
Loans Not Past Due	12,037	12,846
Total ending loan balance	12,078	12,887
Nonaccrual Loans	156	165
Loans on Deposit Accounts
Financing Receivable, Recorded Investment, Past Due
Loans Not Past Due	281	274
Total ending loan balance	281	274
Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due	3	4
Loans Not Past Due	4,217	4,350
Total ending loan balance	4,220	4,354
30 - 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	3	4
30 - 59 Days Past Due | Consumer and Other
Financing Receivable, Recorded Investment, Past Due
Total Past Due	3	4
60 - 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,298	1,207
60 - 89 Days Past Due | One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,298	1,207
90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	894	1,630
90 Days or Greater Past Due | One- to four-family residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	853	1,589
90 Days or Greater Past Due | Home Equity Loans and Lines of Credit
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 41	$ 41