Debt (Narrative) (Details) (USD $)	1 Months Ended			3 Months Ended			9 Months Ended			9 Months Ended				9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	1 Months Ended					1 Months Ended			9 Months Ended
	Apr. 30, 2011	May 31, 2010	Apr. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Jun. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Fixed Interest Rate Range 7.333% To 7.715% [Member] 2006 Senior Unsecured Notes [Member]	Sep. 30, 2011 Variable Interest Rate 5.507% [Member] 2006 Senior Unsecured Notes [Member]	Apr. 30, 2011 Maximum [Member]	Apr. 30, 2011 Minimum [Member]	Sep. 30, 2011 2006 Senior Unsecured Notes [Member]	Dec. 31, 2010 2006 Senior Unsecured Notes [Member]	Sep. 30, 2011 Exchangeable Senior Notes [Member]	Dec. 31, 2010 Exchangeable Senior Notes [Member]	Apr. 30, 2011 Term Loans [Member]	Sep. 30, 2011 Term Loans [Member]	Dec. 31, 2010 Term Loans [Member]	Sep. 30, 2011 Revolving Credit Facilities [Member]	Dec. 31, 2010 Revolving Credit Facilities [Member]	Apr. 30, 2010 6.125% Exchangeable Senior Notes [Member]	Apr. 30, 2011 Federal Funds Rate [Member]	Sep. 30, 2011 Interest Rate Contract [Member]	Sep. 30, 2011 Interest Rate Contract One [Member]	Sep. 30, 2011 Interest Rate Contract Two [Member]	Sep. 30, 2010 Parent [Member]
Debt Instrument [Line Items]
Mortgage loan assumed, value				$ 16,000,000			$ 16,000,000
Mortgage loan assumed, term, years							30
Mortgage loan assumed, interest rate							6.20%
Mortgage loan assumed, maturity date							Jan. 01, 2018
Mortgage loan assumed, date prepayment restriction expires							January 1, 2013
Unsecured senior notes, value of private offering	450,000,000
Unsecured senior notes, interest rate	6.875%													2.67%					6.20%					0.50%
Unsecured senior notes, year of maturity	2021
Repaid and terminated term loan facility							237,810,000	216,325,000										150,000,000
Repaid and terminated collateralized term loan	9,000,000
Revolving credit facility, amended	330,000,000
Unsecured revolving credit facility maturity	October 2015
Federal funds rate spread	1.60%											2.40%	1.60%
LIBOR plus spread interest rate	2.60%											3.40%	2.60%
Percentage of outstanding notes purchased from senior unsecured notes offering				86.60%																			93.00%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum														7.333%	7.333%	6.125%	6.125%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum														7.715%	7.871%	9.25%	9.25%
Exchangeable senior notes due date				2013																			2011
Price of principal amount plus accrued and unpaid interest, in percentage				118.40%																			103.00%
Price of principal amount plus accrued and unpaid interest, in amount				84,100,000																			136,300,000
Public offering of common stock net proceeds, after underwriting discount and commissions			279,000,000
Paid off term loan							30,000,000
Closed credit facility		450,000,000
Charge of refinancing activities				10,400,000	300,000		14,200,000	6,600,000
Interest rate swap, amount fixed						65,000,000																					60,000,000
Total senior notes available				649,013,000		125,000,000	649,013,000		369,970,000
Maturity date of interest rate swap						July 2016
Interest rate of interest rate derivative instrument																										5.507%	5.675%
Fair value of the interest rate swaps																									12,000,000
Hedge ineffectiveness and income statement effect in period				0	0		0	0
Dividend rate as percentage of normalized adjusted funds from operations							95.00%
Debt face amount														125,000,000	125,000,000	20,175,000	91,175,000		14,486,000	157,683,000	39,600,000
Portion of debt instrument face amount										$ 60,000,000	$ 65,000,000
Fixed rate, minimum														7.333%	7.333%	6.125%	6.125%
Fixed rate, maximum														7.715%	7.871%	9.25%	9.25%