Significant Acquisitions (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 property sqft	Sep. 30, 2012
Significant Acquisitions
Number of properties acquired			3
Rental area of properties acquired (in square feet)			1,951,000
Purchase price of property acquired			$ 558,000,000
Acquisition costs			556,000
Estimated fair value of assets acquired at the date of acquisition
Real estate assets	454,447,000		454,447,000
Value of acquired real estate leases	110,039,000		110,039,000
Acquired unfavorable leases	(9,897,000)		(9,897,000)
Total	554,589,000		554,589,000
Pro forma operating results for the company and acquisitions
Revenue	61,300,000	53,895,000	178,322,000	155,747,000
Income from continuing operations	4,080,000	2,539,000	8,175,000	7,687,000
Net income	4,178,000	(11,930,000)	8,469,000	(7,176,000)
Weighted average shares outstanding	100,187	82,937	91,720	82,937
Income from continuing operations per share (in dollars per share)	$ 0.04	$ 0.03	$ 0.09	$ 0.09
Net income per share (in dollars per share)	$ 0.04	$ (0.14)	$ 0.09	$ (0.09)
Revenue recognized due to the acquisitions			14,200,000
Net income from operations recognized due to the acquisitions			$ 1,400,000
Office property in Atlanta, Georgia
Significant Acquisitions
Number of properties acquired			1
Office property in Denver, Colorado
Significant Acquisitions
Number of properties acquired			2