Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended					1 Months Ended					3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended
	Mar. 31, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Real estate properties held for sale property	Dec. 31, 2012 Real estate properties held for sale	Oct. 31, 2013 5.75% Senior Notes due February 15, 2014	Mar. 31, 2013 5.75% Senior Notes due February 15, 2014	Mar. 31, 2013 6.40% Senior Notes due February 15, 2015	Mar. 31, 2013 5.75% Senior Notes due November 1, 2015	Mar. 31, 2013 6.25% Senior Notes due August 15, 2016	Sep. 30, 2013 Unsecured revolving credit facility	Sep. 30, 2012 Unsecured revolving credit facility	Sep. 30, 2013 Unsecured revolving credit facility	Sep. 30, 2012 Unsecured revolving credit facility	Sep. 30, 2013 Mortgage notes maturing from 2014 through 2026 property	Sep. 30, 2013 Unsecured term loan	Sep. 30, 2012 Unsecured term loan	Sep. 30, 2013 Unsecured term loan	Sep. 30, 2012 Unsecured term loan	Sep. 30, 2013 Unsecured revolving credit facility and unsecured term loan	Jun. 19, 2013 Unsecured revolving credit facility and unsecured term loan
Indebtedness
Principal	$ 670,295								$ 145,612	$ 152,560	$ 111,227	$ 260,896
Purchase Price	726,151								148,746	164,140	121,047	292,218
Loss on early extinguishment of debt	(60,027)	(220)	(60,027)	(287)
Prepayment of debt								99,043
Maximum borrowing capacity													750,000		750,000
Maximum amount of borrowing													1,500,000		1,500,000
Maturity date extension period available															1 year
Interest rate stated percentage								5.75%	5.75%	6.40%	5.75%	6.25%
Increased maximum borrowing capacity available under specified circumstances																		1,000,000		1,000,000
Variable rate basis															LIBOR					LIBOR
Basis points on variable rate (as a percent)															1.50%					1.85%
Facility fee of lending commitments under our revolving credit facility (as a percent)															0.35%
Interest rate payable (as a percent)															1.70%					2.00%
Average interest rate (as a percent)													1.70%	1.50%	1.70%	1.50%		2.00%	1.80%	2.10%	1.80%
Amount outstanding													235,000		235,000
Available Borrowing Capacity													515,000		515,000
Loan maturing in December 2016																		500,000		500,000
Number of Real Estate Properties Secured by Mortgage						2											14
Number of buildings secured by mortgage						3											19
Cost of properties mortgaged			5,631,070		7,829,409	631,057	164,041										1,269,286
Mortgaged properties aggregate net book value			4,757,038		6,821,803												1,116,162
Mortgage notes payable, net			$ 925,120		$ 984,827	$ 20,127											$ 925,120
Percentage of principal amount at which the potential acquiring entity may offer to repurchase the note																							51.00%
Ratio of lenders by amount required for waivers of defaults under debt																						0.6667