Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Geographic and Asset type concentration (Details)	12 Months Ended
	Dec. 31, 2017
Florida | Geographic Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	28.10%
TEXAS | Geographic Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	13.90%
TUNISIA | Geographic Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	21.30%
OHIO | Geographic Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	13.10%
GEORGIA, REPUBLIC OF | Geographic Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	22.60%
INDIA | Geographic Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	1.00%
Multifamily | Real Estate Asset Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	54.70%
Student Housing | Real Estate Asset Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	35.70%
medical office building [Member] | Real Estate Asset Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	9.60%