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Increase in revenue due to amortization of deferred rent.

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Increase in shares due to amendments in plan.

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Increase in shares due to amendments in plan, shares solely available for options and share appreciation rights.

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Options awarded, term.

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TenantImprovementsUsefulLives

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Represents the compensation cost capitalized during the period arising from equity-based compensation arrangements (for example, shares of stock, units, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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This item represents a non-contingent liability for the fair value of an obligation to stand ready to perform over the term of a guaranty issued in the event that specified triggering events or conditions occur.

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Amount of the adjustment to rental revenues to measure escalating leasing revenues on a straight line basis.

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Summary of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 02, 2010
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Increased revenue due to amortization of deferred rent	$ 2.4	$ 2.4	$ 2.4
Decrease in revenue due to amortization of lease incentives	1.5	0.7	0.8
Increase in shares due to amendments in plan, shares solely available for options and share appreciation rights	2,684,795			3,600,000
Revenue increased by straight-line rent adjustment	13.7us-gaap_StraightLineRentAdjustments	17.7us-gaap_StraightLineRentAdjustments	21.1us-gaap_StraightLineRentAdjustments
Increase in Common shares available for future awards				6,000,000
Common shares available for future awards	4,304,006
Options awarded, term	one to ten-year
Stock-based compensation expense	6.1	8.3	8.6
Stock-based compensation expense, capitalized	1.7	1.4	2.6
Capitalized interest	4.8us-gaap_InterestCostsCapitalized	2.6us-gaap_InterestCostsCapitalized	2.6us-gaap_InterestCostsCapitalized
Concentration Risk, Percentage	10.00%us-gaap_ConcentrationRiskPercentage1	10.00%us-gaap_ConcentrationRiskPercentage1	10.00%us-gaap_ConcentrationRiskPercentage1
The Grove at Cira Centre South [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Guaranty Liabilities	24.7
PJP VII [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Guaranty Liabilities	0.5
TB-BDN Plymouth Apartments [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Guaranty Liabilities	3.2
Maximum [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Property, Plant and Equipment, Useful Life	55 years 0 months 0 days
TenantImprovementsUsefulLives	16 years 0 months 0 days
Minimum [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Property, Plant and Equipment, Useful Life	5 years 0 months 0 days
TenantImprovementsUsefulLives	1 year 0 months 0 days
Equity Method Investments [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Capitalized interest	2.0	0.6
Accounts receivable and accrued rent receivable [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Concentration Risk, Percentage	10.00%	10.00%
Accrued Rent Receivable [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Allowance for Doubtful Accounts Receivable	$ 13.4	$ 13.0
Tenant relationship value [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Concentration Risk, Percentage	10.00%	10.00%	10.00%