Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended						1 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2013 item	Jun. 30, 2013 item	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Unsecured revolving credit facility	Jun. 30, 2013 Unsecured revolving credit facility	Jul. 31, 2013 Unsecured revolving credit facility	Jun. 30, 2013 Senior unsecured notes 4.30%	Jun. 30, 2013 Senior unsecured notes 6.75%	Jun. 30, 2013 Senior unsecured notes 6.75% maturing in 2021	Jun. 30, 2013 Senior unsecured notes 5.625% due in 2042	Jun. 30, 2013 Mortgages item	Jun. 30, 2013 Mortgages item	Jun. 30, 2013 Mortgages Significant Unobservable Inputs (Level 3)	Jun. 30, 2013 Capital leases item
Indebtedness
Unsecured revolving credit facility, maximum borrowing capacity					$ 750,000	$ 750,000
Number of public issues of unsecured senior notes	4	4
Principal amount of unsecured senior notes	1,092,695	1,092,695		1,092,053				250,000	200,000	300,000	350,000
Aggregate principal amount of mortgage debt												701,094	701,094
Interest rate (as a percent)								4.30%	6.75%	6.75%	5.625%	6.25%	6.25%
Number of properties mortgaged												53	53
Number of properties encumbered under mortgage notes												4	4
Carrying value of mortgaged properties	4,390,563	4,390,563		4,432,404								977,692	977,692		17,984
Mortgage debt assumed in connection with real estate acquisitions		12,266	56,789											12,266
Fair value of mortgage notes payable														13,306
Increase (decrease) in outstanding principal balances														1,040
Number of properties recorded under capital lease															2
Capital leases															13,557
Number of mortgage notes												4
Repayment of mortgage notes												10,377
Extension period of revolving credit facility						1 year
Option to increase the borrowing capacity under revolving credit facility					1,500,000	1,500,000
Interest rate, description						LIBOR
Interest rate added to the base rate (as a percent)					1.60%	1.60%
Debt instrument, facility fee (as a percent)						0.35%
Principal repayment due until maturity						0
Weighted average interest rate on debt (as a percent)					1.85%	1.85%						6.10%	6.10%
Loss on early extinguishment of debt	(105)	(105)											105
Interest expense and other associated costs incurred					74	305
Unsecured revolving credit facility, amount outstanding					30,000	30,000	10,000
Revolving credit facility, available amount					$ 720,000	$ 720,000	$ 740,000