Notes Payable (Narrative) (Details) (USD $)	1 Months Ended	9 Months Ended		3 Months Ended					3 Months Ended		3 Months Ended				1 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended
	Jun. 30, 2011	Sep. 30, 2011 years	Sep. 30, 2010	Jun. 30, 2011 Commercial Banks [Member] Term Loan, Due 2012 [Member]	Sep. 30, 2011 Senior Unsecured Notes [Member] 4.70% Notes Due 2021 [Member]	Jun. 30, 2011 Senior Unsecured Notes [Member] 4.70% Notes Due 2021 [Member]	Sep. 30, 2011 Senior Unsecured Notes [Member] 5.00% Notes Due 2023 [Member]	Jun. 30, 2011 Senior Unsecured Notes [Member] 5.00% Notes Due 2023 [Member]	Mar. 31, 2011 Senior Unsecured Notes [Member] 7.69% Notes Due 2011 [Member]	Sep. 30, 2011 Senior Unsecured Notes [Member] 7.69% Notes Due 2011 [Member]	Jun. 30, 2011 Medium-Term Notes [Member] 4.99% Notes Due 2011 [Member]	Sep. 30, 2011 Medium-Term Notes [Member] 4.99% Notes Due 2011 [Member]	Sep. 30, 2011 Floating Rate Debt [Member]	Sep. 30, 2010 Floating Rate Debt [Member]	Jun. 30, 2011 4.70% Notes Due 2021 [Member]	Sep. 30, 2011 4.70% Notes Due 2021 [Member]	Jun. 30, 2011 5.00% Notes Due 2023 [Member]	Sep. 30, 2011 5.00% Notes Due 2023 [Member]	Sep. 30, 2011 0.95% - 6.00% Conventional Mortgage Notes, Due 2012 - 2045 [Member]	Sep. 30, 2011 0.95% - 6.00% Conventional Mortgage Notes, Due 2012 - 2045 [Member]	Sep. 30, 2011 Line Of Credit [Member]	Sep. 30, 2011 Line Of Credit [Member]	Sep. 30, 2011 Line Of Credit [Member]	Sep. 30, 2011 Letter Of Credit [Member]
Amount of current borrowing capacity under unsecured credit facility																						$ 500,000,000	$ 500,000,000
Prior maturity period of unsecured credit facility																						August 2012
Current maturity period of unsecured credit facility																					September 2015
Optional extension period of unsecured credit facility																					September 2016
Maximum term of bid rate loans (days)																							180
Lesser of amount stated or the amount available under the line of credit																							250,000,000
Maximum borrowing capacity under unsecured credit facility																					750,000,000			100,000,000
Outstanding letters of credit under credit facility																								11,600,000
Available amount under unsecured credit facility																						488,400,000	488,400,000
Repayment of senior unsecured notes		500,000,000		500,000,000					88,000,000		35,000,000
Secured third party note payable to one of our fully-consolidated joint ventures																			31,500,000	31,500,000
Investment ownership percentage in fully-consolidated joint venture																				25.00%
Original maturity period of secured third-party note payable																			August 2011
Extended maturity period of secured third-party note payable																			August 2012
Interest rate of notes payable															4.625%		4.875%
Yield to maturity interest rate on notes payable					4.70%		5.00%		7.69%	7.69%	4.99%	4.99%
Aggregate principal face amount of senior unsecured notes	500,000,000					250,000,000		250,000,000
Proceeds from notes payable		495,705,000	57,601,000
Proceeds from notes payable	491,800,000
Notes payable, maturity date															Jun. 01, 2021		Jun. 01, 2023
Discount on notes payable face amount																99.404%		98.878%
Write off of unamortized loan costs				$ 500,000
Weighted average interest rate on floating rate debt		4.80%											1.10%	1.30%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years)		7.0