Debt (Details) (USD $)	3 Months Ended		9 Months Ended					9 Months Ended				1 Months Ended	9 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended	9 Months Ended			3 Months Ended					9 Months Ended		3 Months Ended				1 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Mortgage and Other Secured Loans	Dec. 31, 2010 Mortgage and Other Secured Loans	Sep. 30, 2011 Fixed rate mortgage loans	Dec. 31, 2010 Fixed rate mortgage loans	Sep. 30, 2011 Fixed rate mortgage loan maturing 2034	Dec. 31, 2010 Revolving Construction Facility	Sep. 30, 2011 Variable rate secured loans loan	Sep. 30, 2011 Variable rate secured loans	Dec. 31, 2010 Variable rate secured loans	Sep. 30, 2011 Other construction loan facilities	Dec. 31, 2010 Other construction loan facilities	Sep. 30, 2011 Revolving Credit Facility	Sep. 30, 2011 Revolving Credit Facility Year	Sep. 30, 2011 Revolving Credit Facility Year	Sep. 01, 2011 Revolving Credit Facility	Dec. 31, 2010 Revolving Credit Facility	Sep. 30, 2011 Revolving Credit Facility Prime Rate	Sep. 30, 2011 Revolving Credit Facility Federal Funds Rate	Sep. 30, 2011 Revolving Credit Facility LIBOR 30-day	Sep. 30, 2011 Revolving Credit Facility LIBOR One-Month Rate	Sep. 30, 2011 Term Loan Facility	Sep. 30, 2011 Term Loan Facility Year	Sep. 01, 2011 Term Loan Facility	Sep. 30, 2011 Term Loan Facility Prime Rate	Sep. 30, 2011 Term Loan Facility Federal Funds Rate	Sep. 30, 2011 Term Loan Facility LIBOR 30-day	Sep. 30, 2011 Term Loan Facility LIBOR One-Month Rate	Sep. 30, 2011 Revolving facilities and variable rate secured loans	Sep. 30, 2011 Unsecured notes payable	Dec. 31, 2010 Unsecured notes payable	Sep. 30, 2011 4.25% Exchangeable Senior Notes	Sep. 30, 2010 4.25% Exchangeable Senior Notes	Sep. 30, 2011 4.25% Exchangeable Senior Notes	Sep. 30, 2010 4.25% Exchangeable Senior Notes	Dec. 31, 2010 4.25% Exchangeable Senior Notes	Sep. 30, 2011 3.5% Exchangeable Senior Notes	Sep. 30, 2010 3.5% Exchangeable Senior Notes	Sep. 30, 2011 3.5% Exchangeable Senior Notes	Sep. 30, 2010 3.5% Exchangeable Senior Notes	Sep. 15, 2011 3.5% Exchangeable Senior Notes	Dec. 31, 2010 3.5% Exchangeable Senior Notes
Debt
Maximum Availability															$ 104,900,000		$ 1,000,000,000	$ 1,000,000,000	$ 1,000,000,000	$ 1,000,000,000						$ 500,000,000	$ 500,000,000
Initial amount borrowed																												400,000,000
Carrying Value	2,420,073,000		2,420,073,000		2,323,681,000	1,117,647,000	1,643,005,000	1,055,540,000	1,173,358,000	4,500,000	142,339,000	39,397,000	39,397,000	310,555,000	22,170,000	16,753,000	671,000,000	671,000,000	671,000,000		295,000,000					400,000,000	400,000,000							5,022,000	1,947,000	226,404,000		226,404,000		223,846,000						159,883,000
Additional borrowing capacity available provided there is no default under the agreement																										100,000,000	100,000,000
Stated interest rate (as a percent)																																		0.00%		4.25%		4.25%								3.50%
Stated Interest Rates, low end of range (as a percent)								5.20%
Stated Interest Rates, high end of range (as a percent)								7.87%
Description of variable rate basis													LIBOR		LIBOR			LIBOR	LIBOR			prime rate	Federal Funds Rate	LIBOR	LIBOR rate for a one-month interest period	LIBOR			Prime rate	Federal Funds Rate	LIBOR	LIBOR rate for a one-month interest period
Additional variable rate, spread low end of range (as a percent), based on leverage levels																						0.75%	0.75%		0.75%				0.65%	0.65%		0.65%
Additional variable rate, spread high end of range (as a percent), based on leverage levels																						1.50%	1.50%		1.50%				1.40%	1.40%		1.40%
Variable rate, spread (as a percent)												2.25%	2.25%		2.75%								0.50%		1.00%					0.50%		1.00%
Variable rate, spread low end of range (as a percent)																	1.75%	1.75%	1.75%					1.75%		1.65%	1.65%				1.65%
Variable rate, spread high end of range (as a percent)																	2.50%	2.50%	2.50%					2.50%		2.40%	2.40%				2.40%
Unamortized discount included in carrying value																																		1,900,000	1,100,000	13,600,000		13,600,000		16,200,000						2,600,000
Unamortized premium included in carrying value								2,600,000	3,200,000
Weighted average interest rate (as a percent)								6.01%							2.92%		2.20%	2.20%	2.20%
Extension option period (in years)																		1	1								1
Interest rate on debt (as a percent)												2.47%	2.47%													2.13%	2.13%
Exchange rate per $1000 principal amount (in common shares)																																						$ 20.8318
Convertible debt principal amount, basis for exchange																																				1,000		1,000
Exchange price per common share based on exchange rate (in dollars per share)																																				$ 48.00		$ 48.00
Repurchase of notes, percentage of principal paid																																													100.00%
Principal amount of debt																												400,000,000								240,000,000		240,000,000							162,500,000	162,500,000
Effective interest rate, including amortization of issuance costs (as a percent)																																				6.05%		6.05%							5.97%
Interest expense at stated interest rate																																				2,550,000	2,550,000	7,650,000	4,930,000		1,169,000	1,421,000	4,013,000	4,265,000
Interest expense associated with amortization of discount			4,778,000	4,360,000																																866,000	815,000	2,558,000	1,618,000		664,000	941,000	2,617,000	2,781,000
Total interest expenses																																				3,416,000	3,365,000	10,208,000	6,548,000		1,833,000	2,362,000	6,630,000	7,046,000
Expansion right, maximum borrowing capacity																	1,500,000,000
Amount available as a percentage of unencumbered asset value (as a percent)																			60.00%
Amount of letter of credit subfacility																	100,000,000	100,000,000	100,000,000
Amount of swingline facility																	100,000,000	100,000,000	100,000,000
Extension fee as a percentage of total availability of the facility (as a percent)																			0.20%								0.20%
Percentage of quarterly fee, low end of range (as a percent)																			0.25%								0.25%
Percentage of quarterly fee, high end of range (as a percent)																			0.35%								0.35%
Remaining borrowing capacity																	323,100,000	323,100,000	323,100,000
Number of loans repaid upon entry into Revolving Credit Facility and Term Loan Agreement												2
Amount of loans repaid upon entry into Revolving Credit Facility and Term Loan Agreement												270,300,000
Loss on early extinguishment of debt	(1,655,000)		(1,680,000)																														1,700,000
Capitalized interest costs	$ 4,500,000	$ 3,900,000	$ 13,100,000	$ 12,000,000