Note 2 - Summary of Significant Accounting Policies (Details Textual) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Contract with Customer, Liability, Total		$ 0		$ 0
Revenues, Total		293,403	$ 297,176	597,481	$ 590,764
Disposal Group, Deferred Gain on Disposal							$ 8,100
Contract with Customer, Asset, Net, Total		0		0
Other Rental Property Income [Member]
Revenues, Total		$ 5,526	$ 6,098	11,112	$ 9,649
AOCI Attributable to Parent [Member]
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax, Total							1,100
Investments in and Advances to Real Estate Joint Ventures [Member]
Disposal Group, Deferred Gain on Disposal							6,900
Other Liabilities [Member]
Disposal Group, Deferred Gain on Disposal							1,200
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption						$ (1,100)	(1,136)	[1]
Accounting Standards Update 2017-05 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]						8,098
Accounting Standards Update 2017-05 [Member] | Gain on Sale of Operating Properties/ Change in Controls of Interests [Member]
Deconsolidation, Gain (Loss), Amount				6,800
Accounting Standards Update 2017-05 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption						$ 8,100	$ 8,098	[1]
Three Months Ended June 30, 2017 [Member] | Reclassification of Reimbursement Income From Revenues From Rental Properties [Member]
Prior Period Reclassification Adjustment				61,000
Three Months Ended June 30, 2017 [Member] | Reclassification of Other Rental Property Income From Revenues From Properties [Member]
Prior Period Reclassification Adjustment				6,100
Six Months Ended June 30, 2017 [Member] | Reclassification of Reimbursement Income From Revenues From Rental Properties [Member]
Prior Period Reclassification Adjustment				119,100
Six Months Ended June 30, 2017 [Member] | Reclassification of Other Rental Property Income From Revenues From Properties [Member]
Prior Period Reclassification Adjustment				9,600
Six Months Ended June 30, 2017 [Member] | Reclassification of Tax Withholdings for Surrender of Restricted Common Stock From Net Cash Flow Provided by Operating Activiites to Net Cash Flow Used for Financing Activities [Member]
Prior Period Reclassification Adjustment				5,300
Reclassification of Personnel Costs Related to Property Management and Services From General Administrative to Operating and Maintenance [Member] | Three Months Ended June 30, 2017 [Member]
Prior Period Reclassification Adjustment				7,300
Reclassification of Personnel Costs Related to Property Management and Services From General Administrative to Operating and Maintenance [Member] | Six Months Ended June 30, 2017 [Member]
Prior Period Reclassification Adjustment				$ 15,600

[1] Represents the impact of change in accounting principles for its respective Accounting Standard Updates ("ASU"). See Footnote 2 of the Notes to Condensed Consolidated Financial Statements for additional disclosure.