Real Estate Investments (Unconsolidated Joint Ventures) (Narrative) (Details) ft² in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) ft² property joint_venture	Sep. 30, 2015 USD ($) property	Sep. 30, 2016 USD ($) ft² property joint_venture	Sep. 30, 2015 USD ($) property	Dec. 31, 2015 USD ($) ft² property joint_venture
Schedule of Equity Method Investments [Line Items]
Aggregate equity investments	$ 24,711,000		$ 24,711,000		$ 56,824,000
Net (loss) income			$ 488,000	$ 1,611,000
Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Number of properties owned | property	643		643
Aggregate balance outstanding	$ 2,813,829,000		$ 2,813,829,000		3,039,882,000
Unconsolidated Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate equity investments	18,600,000		18,600,000		$ 52,800,000
Net (loss) income	200,000	$ 100,000	600,000	$ 1,500,000
Unconsolidated Joint Venture [Member] | Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate balance outstanding	46,200,000		46,200,000
Unconsolidated Joint Venture [Member] | Mortgages [Member] | Recourse debt [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate balance 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