Commitments and Contingencies Commitments and Contingencies (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 sqft Y
Long-term Purchase Commitment [Line Items]
Minimum remaining lease period	16
Maximum remaining lease period	90
Leased Area Of Building	708,844
If the Company takes fee title to Two Logan Square upon a foreclosure of its mortgage, the Company has agreed to pay an unaffiliated third party that holds a residual interest in the fee owner of this property	$ 2,900,000
Business Acquisition, Cost of Acquired Entity, Liabilities Incurred	700,000
Balance Portion Of Acquisition Related Liability	1,500,000
Three Logan Square [Member] | Ground Lease [Member]
Long-term Purchase Commitment [Line Items]
Annual rental payment during initial term of ground lease	10
Minimum Amount Of Annual Rental Payment	3,000,000
Minimum Amount Of Annual Rental Payments	4,000,000
Capital Leases, Future Minimum Payments Due	$ 27,400,000