Property Transactions (Narrative) (Details) (USD $)	6 Months Ended		3 Months Ended	6 Months Ended		1 Months Ended	6 Months Ended	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Terminus Two Hundred LLC [Member]	Jun. 30, 2013 816 Congress [Member]	Apr. 25, 2013 816 Congress [Member] sqft	Feb. 28, 2013 Terminus Office Holdings [Member]	Jun. 30, 2013 Terminus Office Holdings [Member]	Jun. 30, 2013 Terminus One Hundred LLC [Member]	Jun. 30, 2013 Post Oak Central [Member]	Feb. 28, 2013 Post Oak Central [Member] sqft	Feb. 28, 2013 MSREF Terminus Two Hundred LLC [Member] Terminus Two Hundred LLC [Member]
Real Estate, Write-down or Reserve [Line Items]
Ownership Percentage of Partner in Joint Venture						50.00%					80.00%
Business Acquisition, Cost of Acquired Entity, Purchase Price					$ 102,400,000					$ 230,900,000
Business Acquisition, Cost of Acquired Entity, Cash Paid											53,800,000
Outstanding Amount of Mortgage Note Payable of Joint Venture											74,600,000
Gain (Loss) on Disposition of Assets			19,700,000					37,100,000
Proceeds from Sale of Real Estate Held-for-investment	116,006,000	63,236,000				112,200,000
Business Combination, Acquisition Related Costs				342,000					231,000
Debt Instrument, Face Amount							82,000,000
Proceeds from Equity Method Investment, Dividends or Distributions	$ 2,942,000	$ 9,857,000					$ 39,200,000
Square Footage of Real Estate Property					435,000					1,300,000