Property Transactions Property Transactions (Narrative) (Details) (USD $)	3 Months Ended			1 Months Ended	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Terminus Two Hundred LLC [Member]	Feb. 28, 2013 Terminus Office Holdings [Member]	Mar. 31, 2013 Terminus Office Holdings [Member]	Mar. 31, 2013 Terminus One Hundred LLC [Member]	Mar. 31, 2013 Post Oak Central [Member]	Feb. 28, 2013 Post Oak Central [Member] sqft	Apr. 25, 2013 Subsequent Event [Member] 816 Congress [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								$ 230,900,000	$ 102,400,000
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	112,651,000	0		112,200,000
Business Combination, Acquisition Related Costs					122,000		373,000
Debt Instrument, Face Amount					82,000,000
Proceeds from Equity Method Investment, Dividends or Distributions	$ 1,417,000	$ 1,550,000			$ 39,200,000
Square Footage of Real Estate Property								1,300,000.0	434,000