Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended								1 Months Ended																						1 Months Ended		12 Months Ended	1 Months Ended																	12 Months Ended										1 Months Ended				1 Months Ended	12 Months Ended	1 Months Ended
	Dec. 31, 2011 property	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Unsecured revolving credit facility, due October 2015 Y	Dec. 31, 2010 Unsecured revolving credit facility, due October 2015	Dec. 31, 2011 Unsecured term loan, due December 2012	Dec. 31, 2011 Unsecured term loan, due December 2016	Dec. 31, 2010 Unsecured term loan, due December 2016	Jan. 31, 2012 Senior Notes, due 2012 at 6.95%	Dec. 31, 2011 Senior Notes, due 2012 at 6.95%	Dec. 31, 2010 Senior Notes, due 2012 at 6.95%	Dec. 31, 2011 Senior Notes, due 2013 at 6.50%	Dec. 31, 2010 Senior Notes, due 2013 at 6.50%	Dec. 31, 2011 Senior Notes, due 2014 at 5.75%	Dec. 31, 2010 Senior Notes, due 2014 at 5.75%	Dec. 31, 2011 Senior Notes, due 2015 at 6.40%	Dec. 31, 2010 Senior Notes, due 2015 at 6.40%	Dec. 31, 2011 Senior Notes, due 2015 at 5.75%	Dec. 31, 2010 Senior Notes, due 2015 at 5.75%	Dec. 31, 2011 Senior Notes, due 2016 at 6.25%	Dec. 31, 2010 Senior Notes, due 2016 at 6.25%	Dec. 31, 2011 Senior Notes, due 2017 at 6.25%	Dec. 31, 2010 Senior Notes, due 2017 at 6.25%	Dec. 31, 2011 Senior Notes, due 2018 at 6.65%	Dec. 31, 2010 Senior Notes, due 2018 at 6.65%	Dec. 31, 2011 Senior Notes, due 2019 at 7.50%	Dec. 31, 2010 Senior Notes, due 2019 at 7.50%	Dec. 31, 2011 Senior Notes, due 2020 at 5.875%	Dec. 31, 2010 Senior Notes, due 2020 at 5.875%	Dec. 31, 2011 Mortgage notes maturing from 2012 through 2027 property	Jun. 30, 2011 Mortgage Notes Payable, due 2011 at 7.435%	Dec. 31, 2010 Mortgage Notes Payable, due 2011 at 7.435%	Dec. 31, 2011 Mortgage Notes Payable, due 2012 at 7.31%	Jul. 31, 2011 Mortgage Notes Payable, due 2012 at 8.05%	Dec. 31, 2010 Mortgage Notes Payable, due 2012 at 8.05%	Dec. 31, 2011 Mortgage Notes Payable, due 2012 at 6.0%	Dec. 31, 2010 Mortgage Notes Payable, due 2012 at 6.0%	Dec. 31, 2011 Mortgage Notes Payable, due 2014 at 4.95%	Dec. 31, 2010 Mortgage Notes Payable, due 2014 at 4.95%	Dec. 31, 2011 Mortgage Notes Payable, due 2015 at 5.99%	Dec. 31, 2010 Mortgage Notes Payable, due 2015 at 5.99%	Dec. 31, 2011 Mortgage Notes Payable, due 2015 at 5.78%	Dec. 31, 2011 Mortgage Notes Payable, due 2016 at 5.76%	Dec. 31, 2010 Mortgage Notes Payable, due 2016 at 5.76%	Dec. 31, 2011 Mortgage Notes Payable, due 2016 at 6.03%	Dec. 31, 2010 Mortgage Notes Payable, due 2016 at 6.03%	Dec. 31, 2011 Mortgage Notes Payable, due 2016 at 7.36%	Dec. 31, 2010 Mortgage Notes Payable, due 2016 at 7.36%	Dec. 31, 2011 Mortgage Notes Payable, due 2017 at 5.67%	Dec. 31, 2011 Mortgage Notes Payable, due 2017 at 5.68%	Dec. 31, 2011 Mortgage Notes Payable, due 2019	Dec. 31, 2010 Mortgage Notes Payable, due 2019	Dec. 31, 2011 Mortgage Notes Payable, due 2022 at 6.75%	Dec. 31, 2010 Mortgage Notes Payable, due 2022 at 6.75%	Dec. 31, 2011 Mortgage Notes Payable, due 2023 at 6.14%	Dec. 31, 2010 Mortgage Notes Payable, due 2023 at 6.14%	Dec. 31, 2011 Mortgage Notes Payable, due 2026 at 5.71%	Dec. 31, 2010 Mortgage Notes Payable, due 2026 at 5.71%	Dec. 31, 2011 Mortgage Notes Payable, due 2027 at 6.06%	Dec. 31, 2010 Mortgage Notes Payable, due 2027 at 6.06%	Jun. 30, 2011 Secured mortgage debt maturing in 2017 Acquisition two	Aug. 31, 2011 Secured mortgage debt maturing in 2017 Acquisition three	Oct. 31, 2011 Term loan lender	Dec. 31, 2010 Term loan	Mar. 31, 2011 Unsecured floating rate senior notes, due March 2011	Dec. 31, 2010 Unsecured floating rate senior notes, due March 2011	Jun. 30, 2011 Mortgage debt maturing in 2012 and 2015 Acquisition one property
Indebtedness
Outstanding indebtedness	$ 3,575,544	$ 3,222,340		$ 100,000		$ 57,000	$ 500,000	$ 400,000		$ 150,680	$ 150,680	$ 190,980	$ 190,980	$ 244,655	$ 244,655	$ 186,000	$ 186,000	$ 250,000	$ 250,000	$ 400,000	$ 400,000	$ 250,000	$ 250,000	$ 250,000	$ 250,000	$ 125,000	$ 125,000	$ 250,000	$ 250,000			$ 29,421	$ 5,428		$ 23,466	$ 4,633	$ 4,795	$ 12,655	$ 12,940	$ 8,540	$ 8,793	$ 9,256	$ 7,833	$ 8,172	$ 41,335	$ 41,600	$ 11,766	$ 12,196	$ 41,275	$ 265,000	$ 175,000	$ 175,000	$ 4,042	$ 4,307	$ 13,530	$ 14,357	$ 8,012	$ 8,367	$ 12,924	$ 13,392						$ 168,219
Less unamortized net premiums and discounts	1,787	(16,274)
Total	3,577,331	3,206,066
Variable rate basis				LIBOR		LIBOR	LIBOR	LIBOR																																											LIBOR	LIBOR											LIBOR			LIBOR
Basis points on variable rate (as a percent)				1.25%																																																											1.50%
Interest rate stated percentage									6.95%	6.95%	6.95%	6.50%	6.50%	5.75%	5.75%	6.40%	6.40%	5.75%	5.75%	6.25%	6.25%	6.25%	6.25%	6.65%		7.50%	7.50%	5.875%	5.875%		7.435%	7.435%	7.31%	8.05%	8.05%	6.00%	6.00%	4.95%	4.95%	5.99%	5.99%	5.78%	5.76%	5.76%	6.03%	6.03%	7.36%	7.36%	5.67%	5.68%			6.75%	6.75%	6.14%	6.14%	5.71%	5.71%	6.06%	6.06%	5.67%	5.68%
Period for which interest on loan payable is fixed by a cash flow hedge (in years)																																																			7 years
Interest rate fixed by a cash flow hedge (as a percent)																																																			5.66%
Repayments on maturity	975,030	1,618,101	632,059																												29,188		5,404																																168,219
Prepayment of debt																																		23,168
Gain (loss) on early extinguishment of debt	(35)	(796)	20,686	(345)																														310
Number of properties mortgaged	516																													23
Number of properties in which mortgage debt was assumed																																																																			4
Debt assumed in connection with acquisition	321,235		9,069																																																										41,275	265,000					14,960
Fair value of mortgages assumed																																																													44,560	278,671
Fair value of debt assumed																																																																			15,894
Interest rate weighted percentage																																																																			6.35%
Maximum borrowing capacity				750,000
Increased maximum borrowing capacity available under specified circumstances				1,500,000
Possible option for extension of tenure of facility (in years)				1
Interest rate added to the base rate before amendment (as a percent)				2.00%																																																											2.00%
Interest rate added to the base rate after amendment (as a percent)				1.25%																																																											1.50%
Revolving credit facility, average interest rate (as a percent)				2.20%	1.80%
Amount outstanding				100,000
Available borrowing capacity				650,000
Principal balance																																																															557,000	400,000
Maximum amount by which maximum borrowing capacity of debt instrument may be increased																																																															1,000,000
Number of lenders who did not commit to the ammended term loan																																																															3
Amount agreed to be repaid representing lenders who did not to commit to amended terms																																																															57,000
Cost of properties mortgaged	7,244,232	6,357,258																												914,400
Mortgaged properties aggregate net book value	6,310,062	5,506,997																												799,787
Mortgage notes	632,301	351,526																												632,301
Debt repaid at par									150,680
Principal payments due
2012	221,946
2013	197,076
2014	262,842
2015	557,920
2016	959,768
Thereafter	1,375,992
Total	$ 3,575,544	$ 3,222,340		$ 100,000		$ 57,000	$ 500,000	$ 400,000		$ 150,680	$ 150,680	$ 190,980	$ 190,980	$ 244,655	$ 244,655	$ 186,000	$ 186,000	$ 250,000	$ 250,000	$ 400,000	$ 400,000	$ 250,000	$ 250,000	$ 250,000	$ 250,000	$ 125,000	$ 125,000	$ 250,000	$ 250,000			$ 29,421	$ 5,428		$ 23,466	$ 4,633	$ 4,795	$ 12,655	$ 12,940	$ 8,540	$ 8,793	$ 9,256	$ 7,833	$ 8,172	$ 41,335	$ 41,600	$ 11,766	$ 12,196	$ 41,275	$ 265,000	$ 175,000	$ 175,000	$ 4,042	$ 4,307	$ 13,530	$ 14,357	$ 8,012	$ 8,367	$ 12,924	$ 13,392						$ 168,219