Mortgages and Notes Payable (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended							12 Months Ended				15 Months Ended		3 Months Ended		12 Months Ended		3 Months Ended
	Sep. 30, 2010	Mar. 31, 2013	Dec. 31, 2012	Feb. 12, 2013 Unsecured Revolving Credit Facility [Member]	Mar. 31, 2013 Line of Credit [Member]	Feb. 12, 2013 Unsecured Term Loan [Member]	Dec. 31, 2012 Secured Term Loan [Member]	Dec. 31, 2007 Five Percent Exchangeable Guaranteed Note [Member]	Dec. 31, 2012 Five Percent Exchangeable Guaranteed Note [Member]	Jan. 31, 2012 Five Percent Exchangeable Guaranteed Note [Member]	Mar. 31, 2013 Secured Term Loan 1 [Member]	Mar. 31, 2012 Secured Term Loan 2 [Member]	Mar. 31, 2013 Secured Term Loan 2 [Member]	Mar. 31, 2013 Six Percent Convertible Guaranteed Note [Member]	Mar. 31, 2012 Six Percent Convertible Guaranteed Note [Member]	Dec. 31, 2012 Six Percent Convertible Guaranteed Note [Member]	Mar. 31, 2011 Six Percent Convertible Guaranteed Note [Member]	Mar. 31, 2013 Other Debt Obligations [Member]	Mar. 31, 2012 Other Debt Obligations [Member]	Mar. 31, 2013 Minimum [Member]	Feb. 12, 2013 Minimum [Member] Unsecured Revolving Credit Facility [Member]	Feb. 12, 2013 Minimum [Member] Unsecured Term Loan [Member]	Mar. 31, 2013 Maximum [Member]	Feb. 12, 2013 Maximum [Member] Unsecured Revolving Credit Facility [Member]	Feb. 12, 2013 Maximum [Member] Unsecured Term Loan [Member]	Mar. 31, 2013 Unsecured Revolving Credit Facility, Expiring January 2019 [Member] Secured Term Loan [Member]	Feb. 12, 2013 Secured Revolving Credit Facility, Expiring January 2015 [Member] Secured Revolving Credit Facility [Member]	Dec. 31, 2012 Condition One [Member] Secured Term Loan [Member]	Dec. 31, 2012 Condition Four [Member] Secured Term Loan [Member]	Mar. 31, 2013 Interest Rate Swap [Member]
Debt Instrument [Line Items]
Mortgages and notes payable		$ 1,268,654	$ 1,415,961
Debt Instrument, Face Amount				300,000		250,000	255,000	450,000			25,000		35,551	41,146		83,896	115,000										300,000
Debt Instrument, Basis Spread on Variable Rate																					1.50%	1.45%		2.05%	2.00%			2.00%	2.85%
Derivative, Average Variable Interest Rate																														1.42%
Derivative, Notional Amount																														255,000
Long-term Debt, Gross																										255,000
Debt Instrument, Interest Rate, Stated Percentage								5.45%
Line of Credit Facility, Remaining Borrowing Capacity					292,356
Letters of Credit Outstanding, Amount					7,644
Gains (Losses) on Extinguishment of Debt From Continuing Operations								44				1,578
Debt Instrument, Maturity Date														Jan. 31, 2030
Percent Of Notes Required To Be Repurchased At The Option Of The Holders On Set Dates	100.00%
Debt Instrument, Convertible, Conversion Ratio														144.2599
Debt Instrument, Convertible, Conversion Price (dollars per share)														$ 6.93
Debt Instrument, Prepaid Amount									387,850
Debt Instrument, Repurchased and Satisfied Amount										62,150
Unamortized discount														(2,655)		(5,769)
Carrying amount of debt component														38,491		78,127
Carrying amount of equity component														(16,677)		3,654
Effective interest rate														8.10%		8.10%				3.60%			8.50%
Debt, Weighted Average Interest Rate		5.50%	5.60%
Aggregate if-converted value in excess of aggregate principal amount														28,895		42,579
Coupon interest														788	1,725
Discount amortization														223	484
Interest Expense														1,011	2,209
Extinguishment of Debt, Gain (Loss), Net of Tax																		$ 363	$ 27