Fair Value Measurements (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Fair value of impaired properties and other assets
Impairment losses	$ 5,857,000		$ 43,678,000		$ 83,213,000
Exit costs included in impairment losses in connection with dispositions	979,000		4,146,000		0
Fair value measurement on a nonrecurring basis | Projects in development or held for future development
Fair value of impaired properties and other assets
Impairment losses			1,900,000
Fair value measurement on a nonrecurring basis | Property Conveyances
Fair value of impaired properties and other assets
Impairment losses	11,000,000
Exit costs included in impairment losses in connection with dispositions	560,000
Fair value measurement on a nonrecurring basis | Other Properties with Shortened Holding Periods
Fair value of impaired properties and other assets
Number of properties, shortened holding period	2
Fair value measurement on a nonrecurring basis | Strategic Reallocation Plan
Fair value of impaired properties and other assets
Impairment losses			19,000,000
Exit costs included in impairment losses in connection with dispositions	419,000
Fair value measurement on a nonrecurring basis | Additional Property Disposed
Fair value of impaired properties and other assets
Impairment losses			5,100,000
Fair value measurement on a nonrecurring basis | Properties, Net
Fair value of impaired properties and other assets
Impairment losses	31,068,000	[1]	62,702,000	[2],[3]
Fair value measurement on a nonrecurring basis | Significant Unobservable Inputs (Level 3)
Fair value of impaired properties and other assets
Exit costs included in impairment losses in connection with dispositions	979,000
Fair value measurement on a nonrecurring basis | Significant Unobservable Inputs (Level 3) | Strategic Reallocation Plan
Fair value of impaired properties and other assets
Exit costs included in impairment losses in connection with dispositions			4,200,000
Fair value measurement on a nonrecurring basis | Significant Unobservable Inputs (Level 3) | Properties, Net
Fair value of impaired properties and other assets
Exit costs included in impairment losses in connection with dispositions			4,200,000
Assets	4,459,000		173,949,000	[2]
Fair value measurement on a nonrecurring basis | Total Estimated Fair Value | Properties, Net
Fair value of impaired properties and other assets
Assets	4,459,000		173,949,000	[2]
Fair value measurement on a nonrecurring basis | Discounted Cash Flow
Fair value of impaired properties and other assets
Discount rate	10.00%	[4]
Terminal capitalization rate	9.50%	[4]
Market rent growth rate	3.00%	[4]	3.00%	[4]
Expense growth rate	3.00%	[4]	3.00%	[4]
Fair value measurement on a nonrecurring basis | Discounted Cash Flow | Significant Unobservable Inputs (Level 3) | Properties, Net
Fair value of impaired properties and other assets
Assets	4,459,000		166,650,000
Fair value measurement on a nonrecurring basis | Discounted Cash Flow | Minimum
Fair value of impaired properties and other assets
Discount rate			10.00%
Terminal capitalization rate			8.70%
Fair value measurement on a nonrecurring basis | Discounted Cash Flow | Maximum
Fair value of impaired properties and other assets
Discount rate			11.00%
Terminal capitalization rate			10.00%
Fair value measurement on a nonrecurring basis | Discounted Cash Flow | Weighted Average
Fair value of impaired properties and other assets
Discount rate			10.30%
Terminal capitalization rate			8.90%
Fair value measurement on a nonrecurring basis | Yield Analysis Valuation Technique
Fair value of impaired properties and other assets
Fair value inputs yield			12.00%	[4]
Fair value inputs market rent rate per square foot			8.50	[4]
Fair value inputs leasing costs per square foot			20.00	[4]
Fair value measurement on a nonrecurring basis | Yield Analysis Valuation Technique | Significant Unobservable Inputs (Level 3) | Properties, Net
Fair value of impaired properties and other assets
Assets			2,356,000
Fair value measurement on a nonrecurring basis | Discontinued Operations | Strategic Reallocation Plan
Fair value of impaired properties and other assets
Impairment losses	15,200,000		23,200,000
Fair value measurement on a nonrecurring basis | Greater Baltimore | Other Properties with Shortened Holding Periods
Fair value of impaired properties and other assets
Impairment losses	5,900,000
Fair value measurement on a nonrecurring basis | Greater Philadelphia
Fair value of impaired properties and other assets
Impairment losses			46,100,000
Future expenditure redevelopment			25,000,000
Fair value measurement on a nonrecurring basis | Colorado Springs | Strategic Reallocation Plan
Fair value of impaired properties and other assets
Impairment losses			$ 6,900,000

[1] Represents aggregate impairment losses on non recurring fair value measurements resulting in impairment losses, excluding exit costs incurred of $979,000.

[2] Reflects balance sheet classifications of assets at time of fair value measurement, excluding the effect of held for sale classifications.

[3] Represents aggregate impairment losses on non recurring fair value measurements resulting in impairment losses, excluding exit costs incurred of $4.2 million.

[4] Only one value applied for this unobservable input.