Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			1 Months Ended
	Mar. 31, 2012 sqft Y property lease	Mar. 31, 2012 Everett, WA Office sqft property	Mar. 31, 2012 Stockton, CA Office sqft	Apr. 30, 2012 Albany, NY Office sqft	Apr. 30, 2012 Madison, WI Office sqft
Real estate properties
Number of properties owned	71
Aggregate investment in properties	$ 1,495,496
Number of leases executed	7
Weighted average lease term (in years)	4.4
Expenditures committed on leases entered into during 2011	296
Committed but unspent tenant related obligations based on executed leases as of December 31, 2011	8,103
Number of properties acquired or agreed to be acquired		2
Purchase price of acquisitions allocated based on the estimated fair values of the acquired assets and assumed liabilities
Square Feet	38,122	111,908	22,012	64,000	56,889
Purchase Price		20,875	8,236	8,525	23,900
Assumption of mortgage debt					$ 19,249