Indebtedness (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended			1 Months Ended				1 Months Ended				12 Months Ended				0 Months Ended	10 Months Ended	12 Months Ended			12 Months Ended					1 Months Ended			1 Months Ended																					1 Months Ended			1 Months Ended											1 Months Ended													1 Months Ended	12 Months Ended										1 Months Ended		1 Months Ended		1 Months Ended	12 Months Ended		1 Months Ended	6 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Aug. 31, 2012 Series C	Dec. 31, 2012 Series C	Dec. 31, 2011 Series C	Dec. 31, 2012 SIR	Oct. 31, 2012 Acquisition	Sep. 30, 2012 Acquisition	May 31, 2012 Acquisition	Jan. 31, 2012 Acquisition	Dec. 31, 2012 Acquisition	Dec. 31, 2012 Acquisition SIR	Dec. 31, 2012 Unsecured revolving credit facility, due October 2015	Dec. 31, 2011 Unsecured revolving credit facility, due October 2015	Mar. 13, 2012 Unsecured revolving credit facility, due October 2015 SIR	Dec. 31, 2012 Unsecured revolving credit facility, due October 2015 SIR	Dec. 31, 2012 Unsecured revolving credit facility, due October 2015 SIR	Feb. 28, 2013 Unsecured revolving credit facility, due October 2015 SIR	Mar. 31, 2012 Unsecured revolving credit facility, due October 2015 SIR	Dec. 31, 2012 Unsecured revolving credit facility, due March 2016	Dec. 31, 2011 Unsecured term loan, due December 2012	Dec. 31, 2012 Unsecured term loan, due December 2016	Dec. 31, 2011 Unsecured term loan, due December 2016	Dec. 31, 2012 Unsecured term loan, due July 2017	Jan. 31, 2012 Senior Notes, due 2012 at 6.95%	Dec. 31, 2012 Senior Notes, due 2012 at 6.95%	Dec. 31, 2011 Senior Notes, due 2012 at 6.95%	Jul. 31, 2012 Senior Notes, due 2013 at 6.50%	Dec. 31, 2012 Senior Notes, due 2013 at 6.50%	Dec. 31, 2011 Senior Notes, due 2013 at 6.50%	Dec. 31, 2012 Senior Notes, due 2014 at 5.75%	Dec. 31, 2011 Senior Notes, due 2014 at 5.75%	Dec. 31, 2012 Senior Notes, due 2015 at 6.40%	Dec. 31, 2011 Senior Notes, due 2015 at 6.40%	Dec. 31, 2012 Senior Notes, due 2015 at 5.75%	Dec. 31, 2011 Senior Notes, due 2015 at 5.75%	Dec. 31, 2012 Senior Notes, due 2016 at 6.25%	Dec. 31, 2011 Senior Notes, due 2016 at 6.25%	Dec. 31, 2012 Senior Notes, due 2017 at 6.25%	Dec. 31, 2011 Senior Notes, due 2017 at 6.25%	Dec. 31, 2012 Senior Notes, due 2018 at 6.65%	Dec. 31, 2011 Senior Notes, due 2018 at 6.65%	Dec. 31, 2012 Senior Notes, due 2019 at 7.50%	Dec. 31, 2011 Senior Notes, due 2019 at 7.50%	Dec. 31, 2012 Senior Notes, due 2020 at 5.875%	Dec. 31, 2011 Senior Notes, due 2020 at 5.875%	Dec. 31, 2012 Senior Notes, due 2042 at 5.75%	Dec. 31, 2012 Mortgage notes maturing from 2014 through 2026 property	Feb. 29, 2012 Mortgage Notes Payable, due 2012 at 7.31%	Dec. 31, 2012 Mortgage Notes Payable, due 2012 at 7.31%	Dec. 31, 2011 Mortgage Notes Payable, due 2012 at 7.31%	Oct. 31, 2012 Mortgage Notes Payable, due 2012 at 6.0%	Dec. 31, 2012 Mortgage Notes Payable, due 2012 at 6.0%	Dec. 31, 2011 Mortgage Notes Payable, due 2012 at 6.0%	Dec. 31, 2012 Mortgage Notes Payable, due 2014 at 4.95%	Dec. 31, 2011 Mortgage Notes Payable, due 2014 at 4.95%	Dec. 31, 2012 Mortgage Notes Payable, due 2015 at 5.99%	Dec. 31, 2011 Mortgage Notes Payable, due 2015 at 5.99%	Dec. 31, 2012 Mortgage Notes Payable, due 2015 at 5.78%	Dec. 31, 2011 Mortgage Notes Payable, due 2015 at 5.78%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 5.235%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 5.689%	Sep. 30, 2012 Mortgage Notes Payable, due 2016 at 5.689% Acquisition SIR	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 5.76%	Dec. 31, 2011 Mortgage Notes Payable, due 2016 at 5.76%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 6.03%	Dec. 31, 2011 Mortgage Notes Payable, due 2016 at 6.03%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 6.29%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 7.36%	Dec. 31, 2011 Mortgage Notes Payable, due 2016 at 7.36%	Dec. 31, 2012 Mortgage Notes Payable, due 2017 at 5.67%	Dec. 31, 2011 Mortgage Notes Payable, due 2017 at 5.67%	Dec. 31, 2012 Mortgage Notes Payable, due 2017 at 5.68%	Dec. 31, 2011 Mortgage Notes Payable, due 2017 at 5.68%	Dec. 31, 2012 Mortgage Notes Payable, due 2017 at 5.95%	Sep. 30, 2012 Mortgage Notes Payable, due 2017 at 5.95% Acquisition SIR	Dec. 31, 2012 Mortgage Notes Payable, due 2019	Dec. 31, 2011 Mortgage Notes Payable, due 2019	Dec. 31, 2012 Mortgage Notes Payable, due 2021 at 5.69%	Dec. 31, 2012 Mortgage Notes Payable, due 2021 at 5.30%	Dec. 31, 2012 Mortgage Notes Payable, due 2022 at 6.75%	Dec. 31, 2011 Mortgage Notes Payable, due 2022 at 6.75%	Dec. 31, 2012 Mortgage Notes Payable, due 2023 at 6.14%	Dec. 31, 2011 Mortgage Notes Payable, due 2023 at 6.14%	Dec. 31, 2012 Mortgage Notes Payable, due 2026 at 5.71%	Dec. 31, 2011 Mortgage Notes Payable, due 2026 at 5.71%	May 31, 2012 Mortgage Notes Payable, due 2027 at 6.06%	Dec. 31, 2011 Mortgage Notes Payable, due 2027 at 6.06%	Jul. 31, 2012 Unsecured senior notes, due 2042 at 5.75%	Dec. 31, 2012 Unsecured senior notes, due 2042 at 5.75%	Dec. 31, 2012 Unsecured term loan	Dec. 31, 2012 Unsecured term loan	Dec. 31, 2011 Unsecured term loan	Jul. 31, 2012 Unsecured term loan SIR	Dec. 31, 2012 Unsecured term loan SIR
Indebtedness
Outstanding indebtedness	$ 4,324,273	$ 3,575,544												$ 297,000	$ 100,000						$ 95,000	$ 57,000	$ 500,000	$ 500,000	$ 350,000			$ 150,680			$ 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	$ 175,000				$ 5,428			$ 4,633	$ 12,356	$ 12,655	$ 8,272	$ 8,540	$ 8,980	$ 9,256	$ 116,000	$ 7,500		$ 7,474	$ 7,833	$ 40,854	$ 41,335	$ 146,264	$ 11,302	$ 11,766	$ 41,275	$ 41,275	$ 265,000	$ 265,000	$ 18,447		$ 174,870	$ 175,000	$ 28,793	$ 40,185	$ 3,759	$ 4,042	$ 12,650	$ 13,530	$ 7,637	$ 8,012		$ 12,924
Less unamortized net premiums and discounts	25,548	1,787
Total	4,349,821	3,577,331
Interest rate stated percentage								5.235%	5.30%	5.69%	6.29%			1.70%			1.50%	1.50%									6.95%			6.50%		5.75%		6.40%		5.75%		6.25%		6.25%		6.65%		7.50%		5.875%		5.75%			7.31%			6.00%		4.95%		5.99%		5.78%		5.235%	5.689%		5.76%		6.03%		6.29%	7.36%		5.67%		5.68%		5.95%				5.69%	5.30%	6.75%		6.14%		5.71%			6.06%		5.75%	2.10%	2.10%			1.80%
Period for which interest on loan payable is fixed by a cash flow hedge																																																																														7 years
Average interest rate mortgages (as a percent)																																																																5.689%													5.95%	5.66%
Prepayment of debt																										150,680			190,980																																																											12,720
Repayments on maturity	1,312,982	975,030	1,618,101																																															5,404			4,507																																							57,000
Gain (loss) on early extinguishment of debt	(1,895)	(35)	(796)
Assumption of mortgage debt	359,213	321,235						116,000	40,328	29,012	147,872	359,212	26,000																																																			7,500													18,500
Fair value of mortgages assumed								123,386	42,490	31,148	160,330																																																					7,947													19,984
Debt issued																																																																																										175,000			500,000
Net proceeds from public offering of unsecured notes																																																																																										169,000
Preferred shares of beneficial interest, shares redeemed				6,000,000	0	6,000,000
Preferred shares, dividend yield (as a percent)				7.125%		7.125%
Redemption of preferred shares				150,000
Maximum borrowing capacity							500,000							750,000			500,000	500,000	750,000	500,000
Maximum amount of borrowing														1,500,000																																																																														1,000,000	1,000,000		700,000
Variable rate basis														LIBOR	LIBOR			LIBOR			LIBOR	LIBOR	LIBOR	LIBOR	LIBOR																																																					LIBOR	LIBOR														LIBOR
Basis points on variable rate (as a percent)														1.50%			1.30%	1.30%																																																																										1.85%	1.85%			1.55%
Facility fee of lending commitments under our revolving credit facility														0.35%				0.30%
Interest rate payable (as a percent)														1.70%																																																																															2.10%
Average interest rate (as a percent)														1.50%	2.20%		1.50%																																																																												1.80%	2.10%		1.80%
Amount outstanding														297,000			95,000	95,000
Available borrowing capacity														453,000			405,000	405,000
Loan maturing in December 2016	1,319,736																																																																																											500,000	500,000
Maturity date extension period available														1 year		1 year
Term of loan																																																																																															5 years
Principal balance																																																																																												557,000	557,000		350,000
Cost of properties mortgaged	7,829,409	7,244,232																																															1,328,287
Mortgaged properties aggregate net book value	6,821,803	6,310,062																																															1,194,799
Mortgage notes payable, net	984,827	632,301																																															984,827
Number of properties mortgaged																																																	25
Principal payments due
2013	8,408																																																																																											57,000	57,000
2014	265,411
2015	757,649
2016	1,319,736																																																																																											500,000	500,000
2017	929,271
Thereafter	1,043,798
Total	$ 4,324,273	$ 3,575,544												$ 297,000	$ 100,000						$ 95,000	$ 57,000	$ 500,000	$ 500,000	$ 350,000			$ 150,680			$ 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	$ 175,000				$ 5,428			$ 4,633	$ 12,356	$ 12,655	$ 8,272	$ 8,540	$ 8,980	$ 9,256	$ 116,000	$ 7,500		$ 7,474	$ 7,833	$ 40,854	$ 41,335	$ 146,264	$ 11,302	$ 11,766	$ 41,275	$ 41,275	$ 265,000	$ 265,000	$ 18,447		$ 174,870	$ 175,000	$ 28,793	$ 40,185	$ 3,759	$ 4,042	$ 12,650	$ 13,530	$ 7,637	$ 8,012		$ 12,924