Debt (Details) (USD $)	6 Months Ended	132 Months Ended				1 Months Ended				1 Months Ended	6 Months Ended	132 Months Ended			6 Months Ended			0 Months Ended			0 Months Ended				6 Months Ended			6 Months Ended			6 Months Ended			0 Months Ended	6 Months Ended			12 Months Ended
	Jun. 30, 2013	Dec. 31, 2020	Dec. 31, 2012	Jun. 30, 2013 Line of Credit [Member]	Dec. 31, 2012 Secured Term Loan [Member]	Jan. 31, 2012 Five Percent Exchangeable Guaranteed Note [Member]	Dec. 31, 2012 Five Percent Exchangeable Guaranteed Note [Member]	Dec. 31, 2007 Five Percent Exchangeable Guaranteed Note [Member]	Jan. 31, 2012 Secured Term Loan 1 [Member]	Jan. 31, 2012 Secured Term Loan 2 [Member]	Jun. 30, 2013 6% Convertible Guaranteed Note [Member]	Dec. 31, 2020 6% Convertible Guaranteed Note [Member]	Dec. 31, 2012 6% Convertible Guaranteed Note [Member]	Dec. 31, 2010 6% Convertible Guaranteed Note [Member]	Jun. 30, 2013 Other Debt Obligations [Member]	Jun. 30, 2012 Other Debt Obligations [Member]	Jun. 30, 2013 Minimum [Member]	Feb. 12, 2013 Minimum [Member] Unsecured Revolving Credit Facility [Member]	Feb. 12, 2013 Minimum [Member] Unsecured Term Loan [Member]	Jun. 30, 2013 Maximum [Member]	Feb. 12, 2013 Maximum [Member] Unsecured Revolving Credit Facility [Member]	Feb. 12, 2013 Maximum [Member] Unsecured Term Loan [Member]	Jun. 30, 2013 Senior Notes Due 2023 [Member] Senior Notes [Member]	Feb. 12, 2013 Secured Revolving Credit Facility, Expiring January 2015 [Member] Secured Revolving Credit Facility [Member]	Jun. 30, 2013 Unsecured Revolving Credit Facility, Expiring February 2018 [Member]	Jun. 30, 2013 Unsecured Revolving Credit Facility, Expiring February 2018 [Member] Unsecured Revolving Credit Facility [Member]	Feb. 12, 2013 Unsecured Revolving Credit Facility, Expiring February 2018 [Member] Unsecured Revolving Credit Facility [Member]	Jun. 30, 2013 Unsecured Revolving Credit Facility, Expiring February 2018 [Member] Minimum [Member]	Jun. 30, 2013 Unsecured Revolving Credit Facility, Expiring February 2018 [Member] Maximum [Member]	Jun. 30, 2013 Unsecured Revolving Credit Facility, Expiring January 2019 [Member] Secured Term Loan [Member]	Jun. 30, 2013 Unsecured Revolving Credit Facility, Expiring January 2019 [Member] Minimum [Member] Secured Term Loan [Member]	Jun. 30, 2013 Unsecured Revolving Credit Facility, Expiring January 2019 [Member] Maximum [Member] Secured Term Loan [Member]	Jun. 30, 2013 Term Loan Facility from Key Bank [Member]	Feb. 12, 2013 Term Loan Facility from Key Bank [Member] Unsecured Term Loan [Member]	Jun. 30, 2013 Term Loan Facility from Key Bank [Member] Minimum [Member]	Jun. 30, 2013 Term Loan Facility from Key Bank [Member] Maximum [Member]	Jun. 30, 2013 Convertible Debt [Member]	Dec. 31, 2012 Condition One [Member] Secured Term Loan [Member]	Dec. 31, 2012 Condition Four [Member] Secured Term Loan [Member]	Jun. 30, 2013 Interest Rate Contract [Member]	Jun. 30, 2013 Interest Rate Swap [Member]
Debt Instrument [Line Items]
Mortgages and notes payable	$ 1,043,934,000		$ 1,415,961,000
Effective interest rate											8.10%		8.10%				3.60%			8.50%
Debt, Weighted Average Interest Rate	5.40%		5.60%
Debt Instrument, Interest Rate, Stated Percentage								5.45%						6.00%									4.25%
Debt Instrument, Percentage of Issuance Price																							99.026%
Debt Instrument, Unamortized Discount (Premium), Net											2,480,000		5,769,000										2,435,000
Debt Instrument, Face Amount					255,000,000			450,000,000	25,000,000	35,551,000	41,146,000		83,896,000	115,000,000									250,000,000	300,000,000		400,000,000	300,000,000							250,000,000
Letters of Credit Outstanding, Amount				7,644,000
Debt Instrument, Basis Spread on Variable Rate																		1.50%	1.45%		2.05%	2.00%			1.15%			0.95%	1.725%		1.50%	2.25%			1.10%	2.10%		2.00%	2.85%
Debt Instrument, Basis Spread on Variable at the End of the Period																														1.75%			1.35%
Line of Credit Facility, Remaining Borrowing Capacity				392,356,000
Debt Instrument, Term																																		5 years
Derivative, Fixed Interest Rate																																								0.73%
Long-term Debt, Gross																																								64,000,000
Derivative, Average Variable Interest Rate																																									1.42%
Derivative, Notional Amount																																									255,000,000
Gains (Losses) on Extinguishment of Debt From Continuing Operations						44,000				1,578,000
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											146.2412
Debt Instrument, Convertible, Conversion Price (dollars per share)											$ 6.84
Convertible notes payable																																					42,750,000
Debt Conversion, Converted Instrument, Shares Issued	6,167,111
Covertible Debt Cash Payments	2,663,000
Debt Satisfaction Charges	10,633,000
Debt Instrument, Prepaid Amount							387,850,000
Debt Instrument, Repurchased and Satisfied Amount						62,150,000
Extinguishment of Debt, Gain (Loss), Net of Tax															$ 12,089,000	$ 29,000