Related Party Transactions and Investments in Non-Consolidated Entities - Equity in losses of investment in non-consolidated REITs (Details) - USD ($)			1 Months Ended		12 Months Ended
	May 29, 2009	Dec. 27, 2007	May 31, 2009	Dec. 31, 2007	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Sponsored REITs
Equity in losses of non-consolidated REITs					$ 3,604,000	$ 831,000	$ 1,451,000
Impairment charge					2,519,000
Distributions received from non-consolidated REITs
Distributions from non-consolidated REITs					1,396,000	1,023,000	107,000
East Wacker
Sponsored REITs
Equity in losses of non-consolidated REITs					428,000	563,000	1,352,000
Preferred shares purchased		965.75		965.75
Percentage of outstanding preferred shares purchased		43.70%		43.70%
Net cost of preferred shares purchased		$ 82,813,000		$ 82,813,000
Offering price of preferred shares purchased				96,575,000
Commissions excluded				7,726,000
Loan fees excluded				5,553,000
Acquisition fees excluded				$ 483,000
Distributions received from non-consolidated REITs
Distributions from non-consolidated REITs					1,289,000	916,000
Grand Boulevard
Sponsored REITs
Equity in losses of non-consolidated REITs					657,000	268,000	99,000
Preferred shares purchased	175.5		175.5
Percentage of outstanding preferred shares purchased	27.00%		27.00%
Net cost of preferred shares purchased	$ 15,049,000		$ 15,049,000
Offering price of preferred shares purchased			17,550,000
Commissions excluded			1,404,000
Loan fees excluded			1,009,000
Acquisition fees excluded			$ 88,000
Distributions received from non-consolidated REITs
Distributions from non-consolidated REITs					$ 107,000	$ 107,000	$ 107,000