Note 12 - Fair Value Measurements (Details Textual)		6 Months Ended
		Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Impairment of Real Estate		$ 30,500,000	$ 31,300,000
Measurement Input, Cap Rate [Member] | Minimum [Member]
Real Estate, Measurement Input		0.085
Measurement Input, Cap Rate [Member] | Maximum [Member]
Real Estate, Measurement Input		0.09
Measurement Input, Discount Rate [Member] | Minimum [Member]
Real Estate, Measurement Input		0.095
Measurement Input, Discount Rate [Member] | Maximum [Member]
Real Estate, Measurement Input		0.1
Estimate of Fair Value Measurement [Member]
Notes Payable, Fair Value Disclosure	[1]	$ 4,338,371,000		$ 4,601,479,000
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]
Notes Payable, Fair Value Disclosure		4,300,000,000		4,600,000,000
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]
Notes Payable, Fair Value Disclosure		$ 0		$ 1,900,000

[1] The Company determined that the valuation of its Senior Unsecured Notes were classified within Level 2 of the fair value hierarchy and its Credit Facility was classified within Level 3 of the fair value hierarchy. The estimated fair value amounts classified as Level 2 as of June 30, 2018 and December 31, 2017, were $4.3 billion and $4.6 billion, respectively. The estimated fair value amounts classified as Level 3 as of June 30, 2018 and December 31, 2017, were $0 and $1.9 million, respectively.