Real Estate Investments (Unconsolidated Joint Ventures) (Narrative) (Details) ft² in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) ft² property joint_venture	Jun. 30, 2015 USD ($) property	Jun. 30, 2016 USD ($) ft² property joint_venture	Jun. 30, 2015 USD ($) property	Dec. 31, 2015 USD ($) ft² property joint_venture
Schedule of Equity Method Investments [Line Items]
Aggregate equity investments	$ 22,435,000		$ 22,435,000		$ 56,824,000
Net (loss) income			$ 276,000	$ 1,503,000
Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Number of properties owned | property	647		647
Debt outstanding	$ 2,881,257,000		$ 2,881,257,000		3,039,882,000
Unconsolidated Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate equity investments	18,700,000		18,700,000		$ 52,800,000
Net (loss) income	100,000	$ 1,300,000	400,000	$ 1,400,000
Unconsolidated Joint Venture [Member] | Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Debt outstanding	46,200,000		46,200,000
Unconsolidated Joint Venture [Member] | Mortgages [Member] | Recourse debt [Member]
Schedule of Equity Method Investments [Line Items]
Debt outstanding	$ 0		$ 0
Unconsolidated Properties [Member] | REI segment [Member]
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated joint ventures | joint_venture	2		2		3
Number of properties owned | property	2	6	2	6	3
Net rentable area (in square feet) | ft²	0.5		0.5		0.9