Property Transactions (Fair Value of Assets and Liabilities Acquired) (Details) (USD $)	12 Months Ended			6 Months Ended		12 Months Ended				12 Months Ended		12 Months Ended					12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2013 816 Congress Austin, TX [Member]	Apr. 25, 2013 816 Congress Austin, TX [Member] sqft	Dec. 31, 2013 Texas Acquisition [Member]	Jul. 19, 2013 Texas Acquisition [Member]	Jul. 19, 2013 777 Main Fort Worth, TX [Member] sqft	Jul. 19, 2013 Greenway Plaza Houston, TX [Member] sqft	Dec. 31, 2013 Post Oak Central [Member]	Feb. 28, 2013 Post Oak Central [Member] sqft	Dec. 31, 2013 Terminus Two Hundred LLC [Member]	Dec. 31, 2013 Term Loan [Member] Texas Acquisition [Member]	Jul. 29, 2013 Term Loan [Member] Texas Acquisition [Member]	Dec. 31, 2012 MSREF Terminus Two Hundred LLC [Member] sqft	Feb. 28, 2013 MSREF Terminus Two Hundred LLC [Member] Terminus Two Hundred LLC [Member]	Dec. 31, 2013 Terminus Office Holdings [Member]	Feb. 28, 2013 Terminus Office Holdings [Member]	Dec. 31, 2013 Post Oak Central [Member]	Dec. 31, 2013 Terminus Two Hundred LLC [Member]	Dec. 31, 2013 816 Congress Austin, TX [Member]	Dec. 31, 2013 Texas Acquisition [Member]	Dec. 31, 2013 2100 Ross Avenue [Member]	Dec. 31, 2013 Promenade Atlanta, GA [Member]
Business acquisition cost of acquired entity purchase price allocation
Land and improvements																			$ 88,406,000	$ 25,040,000	$ 6,817,000	$ 306,563,000	$ 5,987,000	$ 13,439,000
Building																			118,470,000	101,472,000	86,391,000	586,150,000	36,705,000	94,190,000
Tenant Improvements and FF&E																			10,877,000	17,600,000	3,500,000	114,220,000	9,034,000	8,600,000
Business Acquisition Purchase Price Allocation Tangible Assets Other																			0	101,000	0	0	0	0
Business Acquisition Purchase Price Allocation Deferred Rent Receivable																			0	44,000	0	0	0	0
Tangible assets																			217,753,000	144,257,000	96,708,000	1,006,933,000	51,726,000	116,229,000
Intangible Assets:
Above-market leases																			995,000	1,512,000	89,000	4,959,000	3,267,000	3,991,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases	152,830,000	21,637,000																	26,968,000	14,355,000	8,222,000	117,630,000	8,888,000	16,172,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Ground Lease																			0	0	0	2,958,000	0	0
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option																			0	0	2,403,000	0	0	0
Total intangible assets																			27,963,000	15,867,000	10,714,000	125,547,000	12,155,000	20,163,000
Intangible Liabilities:
Below-market leases																			(14,792,000)	(9,273,000)	(2,820,000)	(47,170,000)	(436,000)	(1,659,000)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease																			0	0	(1,981,000)	(2,508,000)	0	0
Business Acquisition Purchase Price Allocation Intangible Liabilities Total																			(14,792,000)	(9,273,000)	(4,801,000)	(49,678,000)	(436,000)	(1,659,000)
Total net assets acquired					102,400,000		1,100,000,000				200,000,000					53,800,000			230,924,000	150,851,000	102,621,000	1,082,802,000	63,445,000	134,733,000
Square footage of real estate property (square feet)					435,000			980,000	4,300,000.0		1,300,000.0				566,000
Debt Instrument, Face Amount														950,000,000				82,000,000
Proceeds from Equity Method Investment, Dividends or Distributions	67,101,000	37,379,000	8,865,000														39,200,000
Debt Instrument, Face Amount, Extension One														150,000,000
Gains (Losses) on Extinguishment of Debt	0	(94,000)	0										2,600,000
Business Combination, Acquisition Related Costs				342,000		4,200,000				231,000							122,000
Ownership Percentage of Partner in Joint Venture																		50.00%
Proceeds from Sale of Real Estate Held-for-investment	178,966,000	273,386,000	143,623,000														112,200,000
Outstanding Amount of Mortgage Note Payable of Joint Venture																74,600,000
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property												$ 19,700,000					$ 37,100,000