Investment in Office Properties - Purchase Price Allocation for TPGI (Details) (USD $)	12 Months Ended		0 Months Ended
	Dec. 31, 2012	Dec. 31, 2013	Dec. 19, 2013 Thomas Properties Group, Inc. [Member]
Assets [Abstract]
Land		$ 43,600,000	$ 97,132,000
Buildings		580,616,000	506,817,000
In place leases			61,562,000
Tenant Improvements			41,697,000
Leasing Commissions			14,935,000
Management Contracts			1,888,000
Condominium Units			19,900,000
Investment in unconsolidated joint ventures			93,539,000
Other Assets			47,257,000
Asset held for sale			353,752,000
Total Assets			1,238,479,000
Liabilities [Abstract]
Below market leases		18,979,000	50,152,000
Accounts Payable and Accrued Expense			66,596,000
Mortgage assumed		58,694,000	335,604,000
Liabilities held for sale			260,293,000
Total Liabilities			712,645,000
Consideration funded prior to closing			80,000,000
Noncontrolling Interest			34,229,000
Equity issued			411,605,000
Total Purchase Price	898,400,000		1,238,479,000
Unamortized premium on mortgage note		$ 15,896,000	$ 16,613