Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		3 Months Ended							3 Months Ended
	Mar. 31, 2013 item	Dec. 31, 2012	Mar. 31, 2013 Unsecured revolving credit facility	May 31, 2013 Unsecured revolving credit facility	Mar. 31, 2013 Senior unsecured notes 4.30%	Mar. 31, 2013 Senior unsecured notes 6.75%	Mar. 31, 2013 Senior unsecured notes 6.75% maturing in 2021	Mar. 31, 2013 Senior unsecured notes 5.625% due in 2042	Mar. 31, 2013 Mortgages item	Mar. 31, 2013 Mortgages Significant Unobservable Inputs (Level 3)	Mar. 31, 2013 Capital leases item
Indebtedness
Unsecured revolving credit facility, maximum borrowing capacity			$ 750,000
Number of public issues of unsecured senior notes	4
Principal amount of unsecured senior notes	1,092,374	1,092,053			250,000	200,000	300,000	350,000
Aggregate principal amount of mortgage debt									714,439
Interest rate (as a percent)					4.30%	6.75%	6.75%	5.625%	6.25%
Number of properties mortgaged									57
Carrying value of mortgaged properties	4,479,229	4,432,404							983,479		18,124
Mortgage debt assumed in connection with real estate acquisitions	12,266									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											13,676
Extension period of revolving credit facility			1 year
Option to increase the borrowing capacity under revolving credit facility			1,500,000
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%
Principal repayment due until maturity			0
Weighted average interest rate on debt (as a percent)			1.70%
Interest expense and other associated costs incurred			231
Unsecured revolving credit facility, amount outstanding			0
Revolving credit facility, available amount			$ 750,000	$ 750,000