Indebtedness (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended			1 Months Ended				1 Months Ended			9 Months Ended		9 Months Ended			0 Months Ended	7 Months Ended	9 Months Ended	1 Months Ended		1 Months Ended							9 Months Ended		1 Months Ended	3 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Aug. 31, 2012 Series C	Sep. 30, 2012 Series C	Dec. 31, 2011 Series C	Sep. 30, 2012 SIR	Sep. 30, 2012 Acquisition	May 31, 2012 Acquisition	Jan. 31, 2012 Acquisition	Sep. 30, 2012 Acquisition property	Sep. 29, 2012 Acquisition	Sep. 30, 2012 Acquisition SIR	Sep. 30, 2012 Unsecured revolving credit facility	Sep. 30, 2011 Unsecured revolving credit facility	Mar. 13, 2012 Unsecured revolving credit facility SIR	Sep. 30, 2012 Unsecured revolving credit facility SIR	Sep. 30, 2012 Unsecured revolving credit facility SIR	Jan. 31, 2012 Senior Notes, due 2012 at 6.95%	Sep. 30, 2012 Mortgage notes maturing from 2012 through 2026 property	Feb. 29, 2012 Mortgage Notes Payable, due 2012 at 7.31%	May 31, 2012 Mortgage Notes Payable, due 2027 at 6.06%	Sep. 30, 2012 Mortgage Notes Payable, due 2017 at 5.95% Acquisition SIR property	Sep. 30, 2012 Mortgage Notes Payable, due 2016 at 5.689% Acquisition SIR	Oct. 31, 2012 Mortgage Notes Payable, due 2012 at 6.0%	Jul. 31, 2012 Senior Notes, due 2013 at 6.50%	Jul. 31, 2012 Unsecured senior notes, due 2042 at 5.75%	Sep. 30, 2012 Unsecured term loan	Sep. 30, 2011 Unsecured term loan	Jul. 31, 2012 Unsecured term loan SIR	Sep. 30, 2012 Unsecured term loan SIR
Indebtedness
Interest rate stated percentage											5.69%	6.29%		5.30%							6.95%		7.31%	6.06%			6.00%	6.50%	5.75%
Gain (loss) on early extinguishment of debt	$ (220)	$ 310	$ (1,895)	$ 310																	$ (67)			$ (1,608)				$ 220
Assumption of mortgage debt			243,212	321,235						40,328	29,012	147,872	359,212		26,000										18,500	7,500
Fair value of mortgages assumed										42,490	31,148	160,330	42,490												19,984	7,947
Repayments on maturity			1,055,681	738,904																			5,404
Prepayment of debt																					150,680			12,720			4,507	190,980
Debt issued																													175,000	557,000
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%	7.125%
Redemption of preferred shares			150,000			150,000
Maximum borrowing capacity									500,000							750,000		500,000
Variable rate basis																LIBOR														LIBOR
Basis points on variable rate (as a percent)																1.25%			1.30%	1.30%										1.50%			1.55%
Facility fee of lending commitments under our revolving credit facility																0.25%				0.30%
Average interest rate (as a percent)																1.50%	2.20%		1.50%											1.80%	2.20%		1.80%
Amount outstanding																160,000			92,000	92,000
Available borrowing capacity																590,000			408,000	408,000
Loan maturing in December 2016																														500,000
Loan maturing in December 2012																														57,000
Maturity date extension period available																1 year		1 year
Term of loan																																5 years
Principal balance																																350,000
Maximum amount of borrowing																1,500,000		1,000,000												1,000,000		700,000
Number of properties acquired or agreed to be acquired													15												2
Average interest rate mortgages (as a percent)																									5.95%	5.689%
Number of properties mortgaged																						25
Cost of properties mortgaged	7,863,071		7,863,071		7,244,232																	1,154,955
Mortgaged properties aggregate net book value	6,817,594		6,817,594		6,310,062																	1,028,581
Mortgage notes payable, net	$ 869,384		$ 869,384		$ 632,301																	$ 869,384