Subsequent Events (Details) (USD $)	3 Months Ended							1 Months Ended	3 Months Ended
	Mar. 31, 2013 property	Mar. 31, 2012 property	Dec. 31, 2012	May 04, 2013 2013 Acquisitons [Member] sqft property	Mar. 31, 2013 Vacant [Member] Real Estate [Member] property	Apr. 30, 2013 At the Market Offering [Member]	Mar. 31, 2013 At the Market Offering [Member]	May 03, 2013 Property Acquisition [Member] property	Mar. 31, 2013 Total Portfolio, As of Document End Date [Member] property
Subsequent Event [Line Items]
Number Of Real Estate Properties, Vacant					1
Real Estate Investment, Aggregate Purchase Price									$ 2,060,731,000
Remaining Equity Offering, At the Market Offering						51,100,000
Number of Real Estate Properties				721
Square feet of acquired property				16,955,023
Business Acquisition, Cost of Acquired Entity, Purchase Price				2,112,123,000
Number of Businesses Acquired	48	54						20	701
Area of Real Estate Property, Acquisitions								233,816
Real Estate Investment Property, at Cost, Acquisitions								51,392,000
Common stock, issued	154,322,183		179,167,112			492,300	61,000
Common stock, $0.01 par value, 240,000,000 shares authorized, 154,322,153 and 179,167,112 issued and outstanding at March 31, 2013 and December 31, 2012, respectively	1,543,000		1,792,000			16.29
Cumulative Offering Proceeds, Net of Cumulative Offering Costs						$ 7,900,000