Loans Receivable and Other Lending Investments, net (Credit Characteristics for Performing Loans) (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	$ 968,872	$ 1,294,821
Senior mortgages
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	805,506	794,400
Corporate/Partnership loans
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	153,346	490,898
Subordinate mortgages
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	10,020	9,523
Real Estate Finance
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	$ 901,936	$ 1,056,944
Weighted Average Risk Ratings	3.22	2.77
Real Estate Finance | Senior mortgages
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	$ 738,570	$ 713,057
Weighted Average Risk Ratings	3.18	2.72
Real Estate Finance | Corporate/Partnership loans
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	$ 153,346	$ 334,364
Weighted Average Risk Ratings	3.39	2.85
Real Estate Finance | Subordinate mortgages
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	$ 10,020	$ 9,523
Weighted Average Risk Ratings	3.50	3.00