Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended			0 Months Ended	3 Months Ended	9 Months Ended						1 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2013 item	Sep. 30, 2012	Sep. 30, 2013 item	Sep. 30, 2012	Dec. 31, 2012	Sep. 03, 2013 Unsecured revolving credit facility	Sep. 30, 2013 Unsecured revolving credit facility	Sep. 30, 2013 Unsecured revolving credit facility	Oct. 30, 2013 Unsecured revolving credit facility	Sep. 30, 2013 Senior unsecured notes 4.30%	Sep. 30, 2013 Senior unsecured notes 6.75%	Sep. 30, 2013 Senior unsecured notes 6.75% maturing in 2021	Sep. 30, 2013 Senior unsecured notes 5.625% due in 2042	Sep. 30, 2013 Mortgages item	Jun. 30, 2013 Mortgages item	Sep. 30, 2013 Mortgages item	Sep. 30, 2013 Mortgages item	Sep. 30, 2013 Mortgages Significant Unobservable Inputs (Level 3)	Sep. 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	1,093,016		1,093,016		1,092,053					250,000	200,000	300,000	350,000
Aggregate principal amount of mortgage debt														684,615		684,615	684,615
Interest rate (as a percent)										4.30%	6.75%	6.75%	5.625%	6.25%		6.25%	6.25%
Number of properties mortgaged														51		51	51
Number of properties encumbered under mortgage notes														2	4	2	2
Carrying value of mortgaged properties	4,389,735		4,389,735		4,304,928									950,736		950,736	950,736		18,528
Mortgage debt assumed in connection with real estate acquisitions			12,266	112,153														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																			13,436
Number of mortgage notes														2	4
Prepayment of mortgage notes														13,579	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		LIBOR
Interest rate added to the base rate (as a percent)						1.60%		1.30%
Debt instrument, facility fee (as a percent)						0.35%		0.30%
Principal repayment due until maturity								0
Weighted average interest rate on debt (as a percent)							1.68%	1.68%						6.90%	6.10%	6.90%	6.90%
Losses on early extinguishment of debt	692	6,349	797	6,349			538	538								154	259
Interest expense and other associated costs incurred							271	555
Unsecured revolving credit facility, amount outstanding							125,000	125,000	115,000
Revolving credit facility, available amount							$ 625,000	$ 625,000	$ 635,000