Real Estate Investments (Unconsolidated Joint Ventures) (Narrative) (Details) ft² in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) ft² property joint_venture	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) ft² property joint_venture
Schedule of Equity Method Investments [Line Items]
Aggregate equity investments	$ 23,445,000		$ 56,824,000
Carrying value of investment	19,637,000
Net income	$ 206,000	$ 28,000
Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Number of properties owned | property	652
Debt outstanding	$ 2,967,303,000		3,039,882,000
Unconsolidated Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate equity investments	19,600,000		$ 52,800,000
Net income	300,000	$ 100,000
Unconsolidated Joint Venture [Member] | Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Debt outstanding	46,300,000
Unconsolidated Joint Venture [Member] | Mortgages [Member] | Recourse debt [Member]
Schedule of Equity Method Investments [Line Items]
Debt outstanding	$ 0
Unconsolidated Properties [Member] | REI segment [Member]
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated joint ventures | joint_venture	2		3
Number of properties owned | property	2		3
Net rentable area (in square feet) | ft²	0.5		0.9