Related Parties (Narrative) (Details) $ in Thousands	1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Nov. 30, 2016 USD ($) joint_venture	Oct. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Feb. 29, 2016 USD ($) center	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) property	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($) center
Related Party Transaction [Line Items]
Net accounts receivable, related parties					$ 1,300		$ 1,300		$ 2,200
Accounts payable and accrued expenses, related parties					400		400		300
Management fees revenues, related parties					1,400	$ 1,200	4,400	$ 3,500
Real estate joint ventures and partnerships - Investments							31,053	$ 49,877
Total assets					4,313,190		4,313,190		$ 4,426,928
Debt, net					$ 2,011,789		$ 2,011,789
Proceeds to acquire businesses, gross				$ 2,500
Number of operating properties sold				1			12
Unconsolidated Real Estate Joint Venture [Member]
Related Party Transaction [Line Items]
Number of operating properties sold | property							1		5
Consolidated Joint Venture [Member]
Related Party Transaction [Line Items]
Number of partner's interest in consolidated joint ventures acquired | joint_venture	2
Real estate joint ventures and partnerships - Investments	$ 3,300
51% Owned Real Estate joint venture [Member]
Related Party Transaction [Line Items]
Equity interest in acquiree, percentage										51.00%
Number of real estate properties | center										3
Total assets										$ 43,700
Debt, net										$ 72,400
Series of Individually Immaterial Business Acquisitions [Member]
Related Party Transaction [Line Items]
Business acquisition, percentage of voting interests acquired			50.00%			50.00%		50.00%
Fair value of our equity interest before business combination			$ 13,500
Gain recognized on equity interest remeasured to fair value			$ 9,000
Business Combination Achieved in Stages [Member]
Related Party Transaction [Line Items]
Business acquisition, percentage of voting interests acquired				49.00%
Gain recognized on equity interest remeasured to fair value				$ 37,400
Repayments of Accounts Payable [Member] | Corporate Joint Venture [Member]
Related Party Transaction [Line Items]
Payment of payable due to unconsolidated joint venture		$ 4,800
Distribution of Nonmonetary Assets to Groups of Stockholders’ in Lieu of Capital Return Cash Distributions [Member] | Unconsolidated Real Estate Joint Venture [Member]
Related Party Transaction [Line Items]
Recognized gain (loss) associated with remeasurement of land parcel		$ 1,900