Debt (Narrative) (Details) (USD $)	1 Months Ended	12 Months Ended					1 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended				1 Months Ended		1 Months Ended
	Apr. 30, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Mortgages [Member]	Dec. 31, 2011 Unsecured Revolving Credit Facility [Member]	May 31, 2010 Secured Credit Facility [Member]	Apr. 30, 2011 Unsecured Senior Notes [Member]	Dec. 31, 2011 Unsecured Senior Notes [Member]	Sep. 30, 2011 Exchangeable Senior Notes [Member]	Dec. 31, 2011 Exchangeable Senior Notes [Member]	Dec. 31, 2010 Exchangeable Senior Notes [Member]	Dec. 31, 2008 Exchangeable Senior Notes [Member]	Apr. 30, 2008 Exchangeable Senior Notes [Member]	Apr. 30, 2011 Collateralized Term Loan [Member]	Nov. 30, 2008 Collateralized Term Loan [Member]	Apr. 30, 2010 6.125% Exchangeable Senior Notes [Member]	Nov. 30, 2006 6.125% Exchangeable Senior Notes [Member]	Dec. 31, 2011 Minimum [Member] Unsecured Revolving Credit Facility [Member]	Dec. 31, 2011 Maximum [Member] Unsecured Revolving Credit Facility [Member]	Dec. 31, 2011 LIBOR [Member]	Dec. 31, 2011 LIBOR [Member] Minimum [Member]	Dec. 31, 2011 LIBOR [Member] Maximum [Member]	Dec. 31, 2011 Federal Funds Rate [Member] Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Mortgage loan					$ 16,000,000
Interest rate							6.125%		6.375%	9.25%	9.25%			9.25%		5.66%		6.125%
Private offering								450,000,000
New secured credit facility							450,000,000
Credit facility paid		246,262,000	216,765,000	1,232,000				150,000,000							9,000,000
Percentage of outstanding notes purchased from senior unsecured notes offering							93.00%			86.60%		93.00%	85.00%
Interest rate of Senior Notes							6.125%		6.375%	9.25%	9.25%			9.25%		5.66%		6.125%
Paid off term loan							30,000,000
Charge of refinancing activities							6,700,000		14,200,000
Public offering of common stock net proceeds, after underwriting discount and commissions	279,000,000
Credit facilities, amount outstanding						330,000,000										9,000,000
Credit facilities, basis spread of interest rate						1.60%													1.60%	2.40%	2.60%	2.60%	3.40%	0.50%
Fixed rate, minimum												6.125%
Fixed rate, maximum												9.25%
Credit facilities, weighted average interest rate										118.40%
Debt instrument maturity date									Feb. 15, 2022									Nov. 15, 2011
Price of principal amount plus accrued and unpaid interest, in percentage											103.00%		118.50%				103.00%
Price of principal amount plus accrued and unpaid interest, in amount										84,100,000	136,300,000		84,200,000				136,300,000
Debt face amount									$ 200,000,000		$ 11,000,000	$ 91,175,000