Loans Receivable and Allowance for Loan Losses - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) loan	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) loan	Sep. 30, 2018 USD ($) loan	Dec. 31, 2018 USD ($) loan property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held-for-sale	$ 0		$ 0		$ 8,100,000
Proceeds from sales of loans held-for-sale			94,834,000	$ 0
Loans and leases receivable, gain (loss) on sales, net		$ 0
Proceeds from sales of loans receivable		3,700,000	10,273,000	3,695,000
Proceeds from sale of loans receivable, gross gains				15,000
Proceeds from sale of loans receivable, gross losses				0
Purchases of loans receivable	0		$ 29,885,000	$ 4,715,000
Threshold period, past due status of financial receivables			30 days
Threshold period, past due for nonperforming status of financial receivables			90 days		90 days
Financing receivable, nonaccrual	4,297,000		$ 4,297,000		$ 2,789,000
Financing receivable, 90 days or more past due, still accruing	0		0		0
Real estate owned	$ 0		$ 0		$ 92,000
Number of real estate properties acquired through foreclosure | property					1
Mortgage loans in process of foreclosure, number of loans | loan	5		5		14
Mortgage loans in process of foreclosure, amount	$ 866,000		$ 866,000		$ 1,600,000
Specific allowance for loan losses attributable to impaired loans	1,130,000		1,130,000		915,000
Impaired loans for which there are no related allowance for loan losses	9,786,000		9,786,000		11,647,000
Financing receivable, modifications, recorded investment	18,700,000		$ 18,700,000		16,000,000
Number of loans | loan			1	5
Real estate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value of servicing liability	538,200,000		$ 538,200,000		462,700,000
Home equity loans and advances
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, gain (loss) on sales, net	0
Proceeds from sales of loans receivable	2,300,000		4,800,000
Proceeds from sale of loans receivable, gross losses			0
Multifamily and commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, gain (loss) on sales, net	0
Proceeds from sales of loans receivable	5,500,000		5,500,000
Proceeds from sale of loans receivable, gross losses			0
Purchases of loans receivable	24,900,000			$ 2,100,000
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sale of loans receivable, gross gains		15,000
Proceeds from sale of loans receivable, gross losses		0
Purchases of loans receivable		0	5,000,000	$ 2,600,000
Real estate loans | Multifamily and commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	603,000		603,000		154,000
Specific allowance for loan losses attributable to impaired loans	2,000		2,000		0
Impaired loans for which there are no related allowance for loan losses	1,511,000		$ 1,511,000		2,695,000
Number of loans | loan			1	0
Real estate loans | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales of loans held-for-sale	49,100,000		$ 94,100,000
Loans and leases receivable, gain (loss) on sales, net	382,000		710,000
Financing receivable, nonaccrual	1,799,000		1,799,000		819,000
Specific allowance for loan losses attributable to impaired loans	507,000		507,000		537,000
Impaired loans for which there are no related allowance for loan losses	4,420,000		$ 4,420,000		$ 4,156,000
Number of loans | loan			0	4
Less Than 90 Days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, nonaccrual	433,000		$ 433,000
Financial receivable, number of loans in nonaccrual status | loan			7
Freddie Mac
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales of loans receivable	$ 0	$ 0	$ 21,600,000	$ 0