Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		1 Months Ended		3 Months Ended	1 Months Ended			3 Months Ended
	Mar. 31, 2012 sqft	Mar. 31, 2011 sqft	Jan. 31, 2012 Acquisition sqft	Mar. 31, 2011 Acquisition property sqft	Mar. 31, 2012 Acquisition property sqft	May 31, 2012 Pending acquisitions sqft	Apr. 30, 2012 Pending acquisitions property	Jan. 31, 2012 Austin, TX Pending acquisitions Office sqft	Mar. 31, 2012 Hartford, CT Acquisition sqft
Real estate properties information
Number of properties acquired or agreed to be acquired				4	2		2
Property square feet	74,125,000	67,550,000	1,009,940	1,063,364	1,878,335	172,200		172,200	868,395
Aggregate purchase price of properties acquired or agreed to be acquired, excluding closing costs			$ 150,600	$ 136,500	$ 252,100	$ 49,000	$ 104,400	$ 49,000	$ 101,500
Assumption of mortgage debt	147,872		147,872		147,872	29,200		29,200
Funded real estate improvements	23,861
Real estate purchase price allocation, land			30,400	22,614	46,330				15,930
Real estate purchase price allocation, buildings and improvements			115,817	64,104	176,129				60,312
Real estate purchase price allocation, acquired real estate leases			22,189		47,731				25,542
Real estate purchase price allocation, assumed real estate lease obligations			5,348		5,632				284
Real estate purchase price allocation, discount (premium) on mortgage debt			12,458		12,458
Real estate purchase price, reallocation				$ 8,371