Property Transactions (Fair Value of Assets and Liabilities Acquired) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 25, 2013 sqft	Jul. 19, 2013	Feb. 28, 2013 sqft
Intangible assets:
Finite-Lived Intangible Asset, Acquired-in-Place Leases	$ 147,360,000	$ 152,830,000
Intangible liabilities:
Proceeds from Equity Method Investment, Dividends or Distributions	10,296,000	67,101,000	37,379,000
Gains (Losses) on Extinguishment of Debt			(94,000)
Proceeds from Sale of Real Estate Held-for-investment	244,471,000	178,966,000	273,386,000
816 Congress Austin, TX [Member]
Intangible liabilities:
Total net assets acquired				102,400,000
Square footage of real estate property (square feet)				435,000
Business Combination, Acquisition Related Costs		342,000
Texas Acquisition [Member]
Intangible liabilities:
Total net assets acquired					1,100,000,000
Business Combination, Acquisition Related Costs	4,200,000
777 Main Fort Worth, TX [Member]
Intangible liabilities:
Square footage of real estate property (square feet)					980,000
Greenway Plaza Houston, TX [Member]
Intangible liabilities:
Square footage of real estate property (square feet)					4,300,000.0
Post Oak Central [Member]
Intangible liabilities:
Total net assets acquired						200,000,000
Square footage of real estate property (square feet)						1,300,000.0
Business Combination, Acquisition Related Costs	231,000
Terminus Two Hundred LLC [Member]
Intangible liabilities:
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property	19,700,000
Term Loan [Member] | Texas Acquisition [Member]
Intangible liabilities:
Gains (Losses) on Extinguishment of Debt	2,600,000
MSREF Terminus Two Hundred LLC [Member]
Intangible liabilities:
Square footage of real estate property (square feet)		566,000
MSREF Terminus Two Hundred LLC [Member] | Terminus Two Hundred LLC [Member]
Intangible liabilities:
Total net assets acquired						53,800,000
Outstanding Amount of Mortgage Note Payable of Joint Venture						74,600,000
Terminus Office Holdings [Member]
Intangible liabilities:
Debt Instrument, Face Amount						82,000,000
Proceeds from Equity Method Investment, Dividends or Distributions	39,200,000
Business Combination, Acquisition Related Costs	122,000
Ownership Percentage of Partner in Joint Venture						50.00%
Proceeds from Sale of Real Estate Held-for-investment	112,200,000
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property	37,100,000
Terminus Two Hundred LLC [Member]
Business acquisition cost of acquired entity purchase price allocation
Land and improvements		25,040,000
Building		101,472,000
Tenant improvements		17,600,000
Business Acquisition Purchase Price Allocation Tangible Assets Other		101,000
Business Acquisition Purchase Price Allocation Deferred Rent Receivable		44,000
Tangible assets		144,257,000
Intangible assets:
Above-market leases		1,512,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases		14,355,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Ground Lease		0
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option		0
Total intangible assets		15,867,000
Business Acquisition Purchase Price Allocation Tangible Liabilities Accounts Payable		0
Intangible liabilities:
Below-market leases		(9,273,000)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease		0
Business Acquisition Purchase Price Allocation Intangible Liabilities Total		(9,273,000)
Total net assets acquired		150,851,000
816 Congress Austin, TX [Member]
Business acquisition cost of acquired entity purchase price allocation
Land and improvements		6,817,000
Building		86,391,000
Tenant improvements		3,500,000
Business Acquisition Purchase Price Allocation Tangible Assets Other		0
Business Acquisition Purchase Price Allocation Deferred Rent Receivable		0
Tangible assets		96,708,000
Intangible assets:
Above-market leases		89,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases		8,222,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Ground Lease		0
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option		2,403,000
Total intangible assets		10,714,000
Business Acquisition Purchase Price Allocation Tangible Liabilities Accounts Payable		0
Intangible liabilities:
Below-market leases		(2,820,000)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease		(1,981,000)
Business Acquisition Purchase Price Allocation Intangible Liabilities Total		(4,801,000)
Total net assets acquired		102,621,000
Texas Acquisition [Member]
Business acquisition cost of acquired entity purchase price allocation
Land and improvements		306,563,000
Building		586,150,000
Tenant improvements		114,220,000
Business Acquisition Purchase Price Allocation Tangible Assets Other		0
Business Acquisition Purchase Price Allocation Deferred Rent Receivable		0
Tangible assets		1,006,933,000
Intangible assets:
Above-market leases		4,959,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases		117,630,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Ground Lease		2,958,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option		0
Total intangible assets		125,547,000
Business Acquisition Purchase Price Allocation Tangible Liabilities Accounts Payable		0
Intangible liabilities:
Below-market leases		(47,170,000)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease		(2,508,000)
Business Acquisition Purchase Price Allocation Intangible Liabilities Total		(49,678,000)
Total net assets acquired		1,082,802,000
Post Oak Central [Member]
Business acquisition cost of acquired entity purchase price allocation
Land and improvements		88,406,000
Building		118,470,000
Tenant improvements		10,877,000
Business Acquisition Purchase Price Allocation Tangible Assets Other		0
Business Acquisition Purchase Price Allocation Deferred Rent Receivable		0
Tangible assets		217,753,000
Intangible assets:
Above-market leases		995,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases		26,968,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Ground Lease		0
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option		0
Total intangible assets		27,963,000
Business Acquisition Purchase Price Allocation Tangible Liabilities Accounts Payable		0
Intangible liabilities:
Below-market leases		(14,792,000)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease		0
Business Acquisition Purchase Price Allocation Intangible Liabilities Total		(14,792,000)
Total net assets acquired		230,924,000
2100 Ross Avenue [Member]
Business acquisition cost of acquired entity purchase price allocation
Land and improvements			5,987,000
Building			36,705,000
Tenant improvements			9,034,000
Business Acquisition Purchase Price Allocation Tangible Assets Other			0
Business Acquisition Purchase Price Allocation Deferred Rent Receivable			0
Tangible assets			51,726,000
Intangible assets:
Above-market leases			3,267,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases			8,888,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Ground Lease			0
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option			0
Total intangible assets			12,155,000
Business Acquisition Purchase Price Allocation Tangible Liabilities Accounts Payable			0
Intangible liabilities:
Below-market leases			(436,000)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease			0
Business Acquisition Purchase Price Allocation Intangible Liabilities Total			(436,000)
Total net assets acquired			63,445,000
Northpark Town Center [Member]
Business acquisition cost of acquired entity purchase price allocation
Land and improvements	24,577,000
Building	274,151,000
Tenant improvements	21,674,000
Business Acquisition Purchase Price Allocation Tangible Assets Other	0
Business Acquisition Purchase Price Allocation Deferred Rent Receivable	0
Tangible assets	320,402,000
Intangible assets:
Above-market leases	2,846,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases	30,159,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Ground Lease	0
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option	0
Total intangible assets	33,005,000
Business Acquisition Purchase Price Allocation Tangible Liabilities Accounts Payable	0
Intangible liabilities:
Below-market leases	(8,018,000)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease	0
Business Acquisition Purchase Price Allocation Intangible Liabilities Total	(8,018,000)
Total net assets acquired	345,389,000
Fifth Third Center [Member]
Business acquisition cost of acquired entity purchase price allocation
Land and improvements	22,863,000
Building	163,649,000
Tenant improvements	16,781,000
Business Acquisition Purchase Price Allocation Tangible Assets Other	1,014,000
Business Acquisition Purchase Price Allocation Deferred Rent Receivable	0
Tangible assets	204,307,000
Intangible assets:
Above-market leases	632,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases	17,096,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Ground Lease	338,000
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option	0
Total intangible assets	18,066,000
Business Acquisition Purchase Price Allocation Tangible Liabilities Accounts Payable	(1,026,000)
Intangible liabilities:
Below-market leases	(9,374,000)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease	0
Business Acquisition Purchase Price Allocation Intangible Liabilities Total	(9,374,000)
Total net assets acquired	$ 211,973,000