Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended							1 Months Ended					1 Months Ended	12 Months Ended		1 Months Ended		1 Months Ended					12 Months Ended				1 Months Ended		1 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Unsecured revolving credit facility	Feb. 19, 2013 Unsecured revolving credit facility	Dec. 31, 2011 Unsecured revolving credit facility	Jun. 30, 2011 Unsecured revolving credit facility	Jan. 31, 2011 Senior unsecured notes 4.30%	Dec. 31, 2012 Senior unsecured notes 4.30%	Dec. 31, 2011 Senior unsecured notes 4.30%	Dec. 31, 2012 Senior unsecured notes 6.75%	Dec. 31, 2011 Senior unsecured notes 6.75%	Dec. 31, 2011 Senior unsecured notes 6.75% maturing in 2021	Dec. 31, 2011 Senior unsecured notes 6.75% maturing in 2021	Dec. 31, 2012 Senior unsecured notes 6.75% maturing in 2021	Jul. 31, 2012 Senior unsecured notes 5.625% due in 2042	Dec. 31, 2012 Senior unsecured notes 5.625% due in 2042	Oct. 31, 2012 Mortgages item	Aug. 31, 2012 Mortgages item Property	Jun. 30, 2012 Mortgages item	Apr. 30, 2012 Mortgages item	Feb. 29, 2012 Mortgages item	Dec. 31, 2012 Mortgages Property	Dec. 31, 2012 Mortgages Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 Capital leases Property item	Dec. 31, 2011 Capital leases	Jan. 31, 2012 Senior unsecured notes 8.625%	Dec. 31, 2011 Senior unsecured notes 8.625%	Dec. 31, 2011 Previous unsecured revolving credit facility	Jun. 30, 2011 Previous unsecured revolving credit facility	Jan. 31, 2011 Previous unsecured revolving credit facility	Dec. 31, 2010 Previous unsecured revolving credit facility
Indebtedness
Unsecured revolving credit facility, maximum borrowing capacity				$ 750,000			$ 750,000																							$ 550,000		$ 550,000
Number of public issues of unsecured senior notes	4
Principal amount of unsecured senior notes	1,092,053	965,770							250,000		200,000				300,000		350,000
Aggregate principal amount of mortgage debt																							705,255
Number of properties mortgaged																			28				56		2
Carrying value of mortgaged properties	4,432,404	4,091,330																					992,645		15,602	16,027
Mortgage debt assumed in connection with real estate acquisitions	121,793	217,317	2,458																					121,793
Fair value of mortgage notes payable																								127,722
Weighted average maturity period of mortgage debt assumed																							4 years 8 months 12 days
Increase (decrease) in outstanding principal balances																								5,929
Number of properties recorded under capital lease																									2
Repayments of borrowings under revolving credit facility																													70,000		120,000
Number of properties secured by mortgage loans																		1		1		1
Capital leases																									13,792
Sale of senior unsecured notes								250,000					300,000	300,000		350,000
Net proceeds from sale of senior unsecured notes								245,354					292,126			338,561
Early repayment of debt variable portion																			199,197
Number of Properties released from mortgage as collateral																			11
Interest rate, description				LIBOR
Interest rate added to the base rate (as a percent)				1.60%
Debt instrument, facility fee (as a percent)				0.35%
Unsecured revolving credit facility, amount outstanding				190,000	0
Revolving credit facility, available amount				560,000
Interest expense and other associated costs incurred	5,733	2,745	1,606
Number of mortgage loans	20																				17
Repayment of mortgage loans	52,254																	4,152		3,140	32,576	12,386
Gain (loss) on extinguishment of debt	(6,349)	(427)	(2,433)																(6,349)											(427)
Repayment of debt at maturity	225,000		98,780																								225,000
Interest rate (as a percent)								4.30%	4.30%	4.30%	6.75%	6.75%	6.75%	6.75%	6.75%	5.625%		6.50%	6.40%	6.07%		6.03%			7.70%	7.70%	8.625%	8.625%				7.875%
Revolving credit facility, interest rate payable (as a percent)	1.80%
Weighted average interest rate on debt (as a percent)	5.84%	5.94%		1.80%		1.70%															6.95%		5.84%
Extension period of revolving credit facility				1 year
Option to increase the borrowing capacity under revolving credit facility				$ 1,500,000