Indebtedness - Activity (Details) (USD $)	12 Months Ended							12 Months Ended												12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2010 4.75% Senior Notes [Member] Y	Dec. 31, 2009 7.1% Mortgages [Member]	Dec. 31, 2011 New Credit Facility [Member]	Dec. 31, 2010 Old $600 Million Credit Facility [Member]	Dec. 31, 2011 Old Credit Facility [Member]	Dec. 31, 2010 Unsecured Debt [Member] 8.5% Senior Notes [Member]	Dec. 31, 2009 Unsecured Debt [Member] 7.75%, 8.50%, 7.25%, 6.375% and 6.625% Senior Notes [Member]	Dec. 31, 2011 Unsecured Debt [Member] Satisfied in Full [Member] 7.25% Senior Notes [Member]	Dec. 31, 2009 Unsecured Debt [Member] Satisfied in Full [Member] 7.75% Senior Notes [Member]	Dec. 31, 2009 Unsecured Debt [Member] Partial Repayment, Prepaid [Member] 8.5% Senior Notes [Member]	Dec. 31, 2009 Unsecured Debt [Member] Partial Repayment, Prepaid [Member] 7.75% Senior Notes [Member]	Dec. 31, 2009 Unsecured Debt [Member] Partial Repayment, Prepaid [Member] 7.3% Senior Notes [Member]	Dec. 31, 2009 Unsecured Debt [Member] Partial Repayment, Prepaid [Member] 6.375% Senior Notes [Member]	Dec. 31, 2009 Unsecured Debt [Member] Partial Repayment, Prepaid [Member] 6.625% Senior Notes [Member]	Dec. 31, 2010 Mortgages [Member] 7.3% Mortgage Loans [Member]	Mar. 31, 2010 Mortgages [Member] 7.3% Mortgage Loans [Member]	Dec. 31, 2009 Medium-term Notes [Member] 8.125% Medium Term Notes [Member]
Debt Instrument [Line Items]
Repayments of Senior Debt											$ 246,500,000
Extinguishment of Debt, Amount									169,700,000			238,600,000	6,900,000	11,400,000	3,500,000	4,900,000	3,500,000	119,300,000		20,000,000
Debt Instrument, Interest Rate, Stated Percentage				4.75%					8.50%		7.25%	7.75%	8.50%	7.75%	7.25%	6.375%	6.625%			8.125%
Debt, Weighted Average Interest Rate	5.80%	6.20%	6.20%		7.10%														7.30%
Debt Instrument, Maturity Date									August 2010			April 2009	August 2010	April 2009	March 2011	August 2012	October 2017			January 2009
Gains (Losses) on Extinguishment of Debt	0	0	1,547,000							1,500,000
Payments of Debt Extinguishment Costs																		1,200,000
Write off of Deferred Debt Issuance Cost																		936,000
Mortgage Loans on Real Estate, New Mortgage Loans					330,300,000
Debt Instrument, Face Amount				350,000,000
Term of Debt				10
Line of Credit Facility, Maximum Borrowing Capacity						$ 500,000,000	$ 600,000,000	$ 500,000,000
Line of Credit Facility, Expiration Date						November 2015		November 2013
Debt Instrument, Basis Spread on Variable Rate						1.075%		2.30%