Property Transactions (Fair Value of Assets and Liabilities Acquired) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Finite-Lived Intangible Asset, Acquired-in-Place Leases	$ 50,006	$ 21,637
816 Congress [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business Acquisition, Purchase Price Allocation, Land	6,817
Business Acquisition Purchase Price Allocation Tangible Assets Building	86,391
Business Acquisition Purchase Price Allocation Tangible Assets Tenant Improvements and FF and E	3,500
Business Acquisition, Purchase Price Allocation, Tangible Assets	96,708
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Above Market Leases	89
Finite-Lived Intangible Asset, Acquired-in-Place Leases	8,222
Purchase Options, Land	2,403
Business Acquisition, Purchase Price Allocation, Intangible Assets Other than Goodwill	10,714
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Lease	(2,820)
Business acquisition purchase price allocation amortizable intangible liabilities above market ground lease	(1,981)
Business acquisition, purchase price allocation, intangible liabilities	(4,801)
Business Acquisition, Cost of Acquired Entity, Purchase Price	102,621
Terminus Two Hundred LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business Acquisition, Purchase Price Allocation, Land	25,040
Business Acquisition Purchase Price Allocation Tangible Assets Building	101,472
Business Acquisition Purchase Price Allocation Tangible Assets Tenant Improvements and FF and E	17,600
Business Acquisition, Purchase Price Allocation, Other Noncurrent Assets	101
Business Acquisition, Purchase Price Allocation, Deferred Income	44
Business Acquisition, Purchase Price Allocation, Tangible Assets	144,257
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Above Market Leases	1,512
Finite-Lived Intangible Asset, Acquired-in-Place Leases	14,355
Business Acquisition, Purchase Price Allocation, Intangible Assets Other than Goodwill	15,867
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Lease	(9,273)
Business Acquisition, Cost of Acquired Entity, Purchase Price	150,851
Post Oak Central [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business Acquisition, Purchase Price Allocation, Land	88,406
Business Acquisition Purchase Price Allocation Tangible Assets Building	118,470
Business Acquisition Purchase Price Allocation Tangible Assets Tenant Improvements and FF and E	10,877
Business Acquisition, Purchase Price Allocation, Other Noncurrent Assets	0
Business Acquisition, Purchase Price Allocation, Deferred Income	0
Business Acquisition, Purchase Price Allocation, Tangible Assets	217,753
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Above Market Leases	995
Finite-Lived Intangible Asset, Acquired-in-Place Leases	26,968
Business Acquisition, Purchase Price Allocation, Intangible Assets Other than Goodwill	27,963
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Lease	(14,792)
Business Acquisition, Cost of Acquired Entity, Purchase Price	$ 230,924