Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Operating real estate, consolidated variable interest entities	$ 1,659,951	$ 1,639,486
Construction-in-progress, consolidated variable interest entities	39,171	43,393
Accumulated depreciation and amortization, consolidated variable interest entities	257,604	247,410
Mortgages payable, consolidated variable interest entities	$ 448,641	$ 460,372
Preferred shares, authorized	15,000,000	15,000,000
Preferred shares, par value	$ 0.01	$ 0.01
Preferred shares, percentage	5.417%
Common shares of beneficial interest, par value	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized	100,000,000	100,000,000
Common shares of beneficial interest, shares issued	73,216,520	73,090,877
Common shares of beneficial interest, shares outstanding	73,216,520	73,090,877
5.417% Series 1 Cumulative Convertible Preferred
Preferred shares, percentage	5.417%	5.417%
Preferred shares, liquidation preference	$ 25	$ 25
Preferred shares, shares issued	399,896	399,896
Preferred shares, shares outstanding	399,896	399,896
5.0% Series C Cumulative Redeemable Preferred
Preferred shares, percentage	5.00%	5.00%
Preferred shares, liquidation preference	$ 25,000	$ 25,000
Preferred shares, shares issued	6,000	6,000
Preferred shares, shares outstanding	6,000	6,000