Risk Management and Use of Financial Instruments (Details) (USD $)	12 Months Ended								12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Unsecured Debt [Member]	Dec. 31, 2010 Unsecured Debt [Member]	Dec. 31, 2011 Unsecured Debt [Member] $325.0M 4.950% Guaranteed Notes due 2018 [Member]	Mar. 31, 2011 Unsecured Debt [Member] $325.0M 4.950% Guaranteed Notes due 2018 [Member]	Dec. 31, 2010 Unsecured Debt [Member] $325.0M 4.950% Guaranteed Notes due 2018 [Member]	Dec. 31, 2009 Interest Rate Swap [Member]	Dec. 31, 2011 Interest Rate Swap [Member] Cash Flow Hedging [Member] Treasury Lock Agreement [Member]	Mar. 31, 2011 Interest Rate Swap [Member] Cash Flow Hedging [Member] Treasury Lock Agreement [Member]	Mar. 31, 2011 Interest Rate Swap [Member] Cash Flow Hedging [Member] Treasury Lock Agreement [Member] 2.891% Treasury Rate [Member]	Mar. 31, 2011 Interest Rate Swap [Member] Cash Flow Hedging [Member] Treasury Lock Agreement [Member] 2.873% Treasury Rate [Member]	Mar. 31, 2011 Interest Rate Swap [Member] Cash Flow Hedging [Member] Treasury Lock Agreement [Member] 2.858% Treasury Rate [Member]
Risk Management and use of Financial Instruments (Textuals) [Abstract]
Long-term Debt, Gross				$ 1,888,111,000	$ 1,722,326,000	$ 325,000,000	$ 325,000,000	$ 0
Debt instrument stated interest rate						4.95%	4.95%
Number of Treasury Lock agreements											7
Fixed Interest Rate												2.891%	2.873%	2.858%
Fair value	3,886,000										600,000
Notional Amount	452,836,000										230,000,000
Remaining maturity											7 years
Accumulated other comprehensive income related to debt offering that will be amortized over the term of the note	(6,079,000)	(1,945,000)								600,000
Fair value adjustment of the hedge charged to expenses									1,100,000
Gain on fair value hedge ineffectiveness			$ 100,000
No. of tenants accounted for 10% or more of the Company's rents	0	0	0