Multifamily Loans - Geographic Concentration of Multifamily Loans (Details) - Multifamily securities	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	100.00%	100.00%
California
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	13.00%	11.00%
Florida
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	13.00%	10.00%
North Carolina
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	9.00%	0.00%
Oregon
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	7.00%	0.00%
Hawaii
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	5.00%	0.00%
Tennessee
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	5.00%	0.00%
Texas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	0.00%	13.00%
Arizona
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	0.00%	6.00%
Georgia
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	0.00%	6.00%
Washington
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	0.00%	5.00%
Colorado
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	0.00%	5.00%
Other states (none greater than 5%)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk percentage	48.00%	44.00%