Loans Receivable and Other Lending Investments, net - Credit Characteristics for Performing Loans (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	$ 849,834	$ 924,387
Senior mortgages
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	694,908	764,532
Corporate/Partnership loans
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	144,556	149,663
Subordinate mortgages
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	10,370	10,192
Real Estate Finance
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	$ 783,334	$ 857,662
Weighted Average Risk Ratings	2.83	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	$ 628,408	$ 697,807
Weighted Average Risk Ratings	2.81	2.76
Real Estate Finance | Corporate/Partnership loans
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	$ 144,556	$ 149,663
Weighted Average Risk Ratings	2.89	2.84
Real Estate Finance | Subordinate mortgages
Recorded Investments in loans, presented by class and by credit quality, as indicated by risk rating
Performing Loans	$ 10,370	$ 10,192
Weighted Average Risk Ratings	3.00	3.00