Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands		3 Months Ended									12 Months Ended
		Dec. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)		Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)
Real Estate Under Development Period After Completion Cessation of Cost Capitalization												1 year
Financing Receivable Past Due Term												90 days
Number of Portfolio Segments		1										1
Capitalized Computer Software, Net, Ending Balance		$ 4,300				$ 6,200						$ 4,300	$ 6,200
Capitalized Computer Software, Amortization												5,300	4,600		$ 8,000
Revenues, Total		$ 284,201	$ 283,080	$ 293,403	$ 304,078	315,225		$ 294,845	$ 297,176	$ 293,588		$ 1,164,762	1,200,834		1,170,792
Disposal Group, Deferred Gain on Disposal						8,100							8,100
Number of Equity Participation Plans		2										2
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												10 years
Restricted Stock [Member] | Share-based Compensation Award, Vesting on Fourth or Fifth Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage												100.00%
Restricted Stock [Member] | Share-based Compensation Award, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												4 years
Restricted Stock [Member] | Share-based Compensation Award, Tranche Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												5 years
Restricted Stock [Member] | Share-based Compensation Award, Vesting After the Fifth Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												10 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage												20.00%
Restricted Stock [Member] | Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												3 years
Prior Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | shares		47,000,000										47,000,000
The 2010 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | shares		10,000,000										10,000,000
Billed Accounts Receivable [Member]
Allowance for Doubtful Accounts Receivable, Ending Balance		$ 10,300				9,200						$ 10,300	9,200
Straight-line Rent Receivable [Member]
Allowance for Doubtful Accounts Receivable, Ending Balance		10,200				7,900						10,200	7,900
Other Rental Property Income [Member]
Revenues, Total												$ 20,877	23,552		$ 20,021
After Issuance [Member]
Redeemable Noncontrolling Units Redemption Period												1 year
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption						(1,136)	[1]						(1,136)	[1]			$ (1,100)
Accounting Standards Update 2017-05 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]					8,098							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						8,098	[1]						8,098	[1]			$ 8,100
AOCI Attributable to Parent [Member]
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax, Total		$ 1,100										$ 1,100
Investments in and Advances to Real Estate Joint Ventures [Member]
Disposal Group, Deferred Gain on Disposal						6,900							6,900
Other Liabilities [Member]
Disposal Group, Deferred Gain on Disposal						$ 1,200							$ 1,200
Minimum [Member] | Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												3 years
Minimum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												1 year
Minimum [Member] | Computer Software, Intangible Asset [Member]
Finite-Lived Intangible Asset, Useful Life												3 years
Minimum [Member] | Accounting Standards Update 2016-02 [Member] | Scenario, Forecast [Member]
Operating Lease, Indirect Cost											$ 11,000
Minimum [Member] | Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Operating Lease, Right-of-Use Asset																$ 80,000
Operating Lease, Liability, Total																80,000
Maximum [Member] | Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												5 years
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												3 years
Maximum [Member] | Computer Software, Intangible Asset [Member]
Finite-Lived Intangible Asset, Useful Life												5 years
Maximum [Member] | Accounting Standards Update 2016-02 [Member] | Scenario, Forecast [Member]
Operating Lease, Indirect Cost											$ 14,000
Maximum [Member] | Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Operating Lease, Right-of-Use Asset																110,000
Operating Lease, Liability, Total																$ 110,000
Building and Building Improvements [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life												5 years
Building and Building Improvements [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life												50 years
Reclassification of Reimbursement Income From Revenues From Rental Properties [Member] | Year Ended December 31, 2017 [Member]
Prior Period Reclassification Adjustment												$ 247,600
Reclassification of Reimbursement Income From Revenues From Rental Properties [Member] | Year Ended December 31, 2016 [Member]
Prior Period Reclassification Adjustment												239,000
Reclassification of Other Rental Property Income From Revenues From Properties [Member] | Year Ended December 31, 2017 [Member]
Prior Period Reclassification Adjustment												23,600
Reclassification of Other Rental Property Income From Revenues From Properties [Member] | Year Ended December 31, 2016 [Member]
Prior Period Reclassification Adjustment												20,000
Reclassification of Personnel Costs Related to Property Management and Services From General Administrative to Operating and Maintenance [Member] | Year Ended December 31, 2017 [Member]
Prior Period Reclassification Adjustment												26,800
Reclassification of Personnel Costs Related to Property Management and Services From General Administrative to Operating and Maintenance [Member] | Year Ended December 31, 2016 [Member]
Prior Period Reclassification Adjustment												30,500
Reclassification of Gain on Sale of Operating Properties/Change in Control of Interests to (Provision)/Benefit for Income Taxes, Net [Member] | Year Ended December 31, 2016 [Member]
Prior Period Reclassification Adjustment												$ 6,000

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