Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended			9 Months Ended				1 Months Ended		0 Months Ended	1 Months Ended				9 Months Ended			1 Months Ended
	Oct. 31, 2012	Sep. 30, 2012 item	Sep. 30, 2012 item	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Unsecured revolving credit facility	Sep. 30, 2012 Senior unsecured notes 4.30%	Sep. 30, 2012 Senior unsecured notes 6.75%	Sep. 30, 2012 Senior unsecured notes 6.75% maturing in 2021	Jul. 31, 2012 Senior unsecured notes 5.625% due in 2042	Sep. 30, 2012 Senior unsecured notes 5.625% due in 2042	Aug. 31, 2012 Mortgages item	Oct. 31, 2012 Mortgages item	Jun. 30, 2012 Mortgages item	Apr. 30, 2012 Mortgages item	Feb. 29, 2012 Mortgages item	Sep. 30, 2012 Mortgages item	Sep. 30, 2012 Mortgages Significant Other Observable Inputs (Level 2)	Sep. 30, 2012 Capital leases item	Jan. 31, 2012 Senior unsecured notes 8.625%
Indebtedness
Unsecured revolving credit facility, maximum borrowing capacity						$ 750,000
Number of public issues of unsecured senior notes		4	4
Principal amount of unsecured senior notes		1,091,732	1,091,732		965,770		250,000	200,000	300,000		350,000
Aggregate principal amount of mortgage debt																	702,636
Number of properties mortgaged												28			17		56
Carrying value of mortgaged properties		4,372,441	4,372,441		4,091,330												975,116		15,667
Mortgage debt assumed in connection with real estate acquisitions	12,300		112,153	76,931														112,153
Fair value of mortgage notes payable																		117,271
Weighted average maturity period of mortgage debt assumed																	4 years 7 months 6 days
Increase (decrease) in outstanding principal balances																		5,118
Number of properties recorded under capital lease																			2
Repayments of borrowings under revolving credit facility																				225,000
Number of properties secured by mortgage loans													1	1		1
Capital leases																			13,900
Sale of senior unsecured notes										350,000
Net proceeds from sale of senior unsecured notes										338,566
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						55,000
Revolving credit facility, available amount						695,000
Number of mortgage loans															17
Repayment of mortgage loans													4,159	3,140	32,576	12,400
Gain (loss) on extinguishment of debt		(6,349)	(6,349)	(427)								(6,349)
Interest rate (as a percent)							4.30%	6.75%	6.75%	5.625%	5.625%	6.40%	6.50%	6.07%		6.03%				8.625%
Weighted average interest rate on debt (as a percent)						1.80%									6.95%		5.77%
Extension period of revolving credit facility						1 year
Option to increase the borrowing capacity under revolving credit facility						$ 1,500,000