Commitments and Contingencies Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 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	$ 2.9
Business Acquisition, Cost of Acquired Entity, Liabilities Incurred	0.7
Balance Portion Of Acquisition Related Liability	$ 1.6

Fair Value of the liabilities incurred by the acquirer to former owners of the acquiree, including the fair value of any contingent consideration. If liabilities transferred as consideration remain within the combined entity after the business combination (for example, because the assets or liabilities were transferred to the acquiree rather than to its former owners), and the acquirer therefore retains control of them; in that situation, the acquirer measures those liabilities at their carrying amounts immediately before the acquisition date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 30

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

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-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

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-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6405-128476

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph f(3)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 39, 40, 41

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details