Debt Obligations (Details Textuals) (USD $)	12 Months Ended			0 Months Ended								0 Months Ended								0 Months Ended			0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 28, 2012 Unsecured Debt [Member]	Dec. 31, 2012 Unsecured Debt [Member]	Dec. 31, 2011 Unsecured Debt [Member]	Dec. 31, 2012 Secured Debt [Member]	Dec. 31, 2011 Secured Debt [Member]	Dec. 31, 2012 Letter of Credit [Member] Unsecured Debt [Member]	Dec. 31, 2012 New Revolving Credit Facility [Member] Unsecured Debt [Member]	Dec. 31, 2011 New Revolving Credit Facility [Member] Unsecured Debt [Member]	Dec. 19, 2012 Note 2023 [Member] Unsecured Debt [Member]	Dec. 31, 2012 Note 2023 [Member] Unsecured Debt [Member]		Dec. 31, 2011 Note 2023 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $300.0M 5.750% Guaranteed Notes due 2012 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $300.0M 5.750% Guaranteed Notes due 2012 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $250.0M 5.400% Guaranteed Notes due 2014 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $250.0M 5.400% Guaranteed Notes due 2014 [Member] Unsecured Debt [Member]	Dec. 28, 2012 $250.0M 7.500% Guaranteed Notes due 2015 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $250.0M 7.500% Guaranteed Notes due 2015 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $250.0M 7.500% Guaranteed Notes due 2015 [Member] Unsecured Debt [Member]	Dec. 28, 2012 $250.0M 6.000% Guaranteed Notes due 2016 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $250.0M 6.000% Guaranteed Notes due 2016 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $250.0M 6.000% Guaranteed Notes due 2016 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $300.0M 5.700% Guaranteed Notes due 2017 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $300.0M 5.700% Guaranteed Notes due 2017 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $325.0M 4.950% Guaranteed Notes due 2018 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $325.0M 4.950% Guaranteed Notes due 2018 [Member] Unsecured Debt [Member]	Dec. 31, 2012 Three Year Term Loan [Member] Unsecured Debt [Member]	Feb. 01, 2012 Three Year Term Loan [Member] Unsecured Debt [Member]	Dec. 31, 2011 Three Year Term Loan [Member] Unsecured Debt [Member]	Dec. 31, 2012 Four Year Term Loan [Member] Unsecured Debt [Member]	Feb. 01, 2012 Four Year Term Loan [Member] Unsecured Debt [Member]	Dec. 31, 2012 Seven Year Term Loan [Member] Unsecured Debt [Member]	Feb. 01, 2012 Seven Year Term Loan [Member] Unsecured Debt [Member]		Dec. 31, 2011 Seven Year Term Loan [Member] Unsecured Debt [Member]	Dec. 31, 2012 Note Nine [Member] Unsecured Debt [Member]	Dec. 31, 2012 Federal Funds Rate [Member] Bank Term Loan and the Credit Facility [Member]
Participating Mortgage Loans [Line Items]
Debt Instrument, Basis Spread on Variable Rate										1.50%																				1.75%					1.90%					0.50%
Properties encumbered mortgage indebtedness carrying value, net							$ 591,300,000	$ 678,000,000
Debt obligations (Textuals)
Debt instrument, face amount																300,000,000		250,000,000			250,000,000			250,000,000		300,000,000		325,000,000		150,000,000	150,000,000		100,000,000	250,000,000	200,000,000	200,000,000	[1]		250,000,000
Letters of credit outstanding									900,000
Unused availability under the Credit Facility										530,100,000
Option for additional borrowings subject to customary conditions and limitations										200,000,000
Derivative, Average Fixed Interest Rate																														2.596%					3.623%
Debt Instrument, Interest Rate, Stated Percentage													3.95%	[2]		5.75%		5.40%			7.50%			6.00%		5.70%		4.95%											3.95%
Amortization of Financing Costs	6,208,000	4,991,000	3,770,000
Refund Of Deferred Financing Costs Related To Forward Commitment	0	0	1,659,000
Long-term Debt, Gross					2,027,953,000	1,888,111,000	443,942,000	512,391,000		69,000,000	0		250,000,000		0	0	151,491,000	238,379,000	242,681,000		166,535,000	227,329,000		150,429,000	250,000,000	300,000,000	300,000,000	325,000,000	325,000,000	150,000,000		0			200,000,000			0
Debt Instrument, Unamortized Discount, Percent of Face Amount												99.273%
Debt Instrument yield to maturity												4.037%
Debt Instrument yield to maturity spread at time of pricing												2.35%
Debt Instrument, Unamortized Discount													1,800,000
Proceeds from Notes Payable	0	0	396,000									246,100,000
Early Repayment of Senior Debt	188,430,000			171,500,000																50,300,000			99,600,000
Gain (loss) on early extinguishment of debt	$ (22,002,000)	$ (2,776,000)	$ (2,110,000)	$ 20,400,000

[1] On February 1, 2012, the Company closed on a new $600.0 million four-year unsecured credit facility and three unsecured term loans (the "New Term Loans") totaling $600.0 which consist of a $150.0 million three-year loan, a $250.0 million four-year loan (with $150.0 million swapped to fixed) and a $200.0 million seven-year loan. The Company used a portion of the net proceeds from the New Term Loans to repay all balances outstanding under its prior revolving credit facility and its prior $183.0 million bank term loan which were then retired prior to their scheduled June 29, 2012 maturity. On December 31, 2012, the Company repaid the entire portion of the four-year loan that was swapped to fixed (see related discussion below).

[2] On December 18, 2012, the Company issued new 3.95% $250.0 million Guaranteed Notes due 2023. The Company used the net proceeds from this debt offering to redeem portions of its 7.500% Guaranteed Notes due May 15, 2015 and 6.000% Guaranteed Notes due April 1, 2016 (see related discussion below).