Investment in Loans (Schedule of Commercial Loans: Geographic Distribution) (Details) - Financing Receivables, Unpaid Principal Balance [Member] - Geographic Concentration Risk [Member] - Commercial mortgage loans	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	100.00%	100.00%
Florida [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	27.60%	23.80%
NEW YORK
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	16.60%	15.20%
Virginia [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	4.10%	4.20%
CONNECTICUT
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	10.90%	11.20%
MASSACHUSETTS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	0.70%	5.80%
NORTH CAROLINA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	7.60%	7.90%
CALIFORNIA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	5.70%	5.90%
MASSACHUSETTS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	5.90%	6.10%
ARIZONA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	4.20%	4.30%
Indiana [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	2.70%	2.80%
PENNSYLVANIA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	2.10%	2.20%
NEVADA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	1.80%	1.90%
ILLINOIS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	1.30%	1.40%
OHIO
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of Total Outstanding Unpaid Principal Balance	8.80%	7.30%