Loans Receivable and Allowance for Loan Losses - Narrative (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) loan	Mar. 31, 2018 USD ($) loan	Dec. 31, 2018 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale	$ 0		$ 8,100,000
Proceeds from sales of loans receivable	9,311,000	$ 0
Gross gains on sale of loans and leases receivable	132,000
Gross losses on sale of loans and leases receivable	0
Purchases of loans receivable	2,313,000	$ 4,715,000
Carrying value of servicing liability	$ 474,700,000		$ 462,700,000
Threshold period past due status of financing receivables	30 days
Threshold period, past due for nonperforming status of financing receivables	90 days		90 days
Financing receivable, recorded investment, past due	$ 13,866,000		$ 18,196,000
Financing receivable, recorded investment, nonaccrual status	6,769,000		2,789,000
Loans past due 90 days and still accruing interest	0		0
Real estate owned	$ 0		$ 92,000
Mortgage loans in process of foreclosure, number of loans | loan	6		14
Mortgage loans in process of foreclosure, amount	$ 963,000		$ 1,600,000
Specific allowance for loan losses attributable to impaired loans	1,342,000		915,000
Impaired loans for which there are no related allowance for loan losses	9,179,000		11,647,000
Financing receivable, modifications, recorded investment	$ 19,700,000		16,000,000
Number of loans | loan	1	2
One Borrower
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, recorded investment, nonaccrual status	$ 2,400,000
Number of loans in nonaccrual status | loan	3
Multifamily and commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchases of loans receivable		$ 2,100,000
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales of loans receivable	$ 17,400,000
Purchases of loans receivable	2,300,000	2,700,000
90 Days or More
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, recorded investment, past due	4,409,000		2,789,000
Financing receivable, modifications, recorded investment			$ 101,000
Number of loans | loan			1
30-59 Days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, recorded investment, past due	6,225,000		$ 12,012,000
Financing receivable, modifications, recorded investment	$ 894,000		$ 1,000,000
Number of loans | loan	4		7
Freddie Mac
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales of loans receivable	$ 6,100,000	$ 0