Property Transactions (Fair Value of Assets and Liabilities Acquired) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 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	$ 162,792	$ 21,637
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 Tangible Assets Other	0
Business Acquisition Purchase Price Allocation Deferred Rent Receivable	0
Business Acquisition Purchase Price Allocation Tangible Assets Total	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 Amortizable Intangible Liabilities Below Market Ground Lease	0
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option	0
Business Acquisition Purchase Price Allocation Intangible Assets Total	27,963
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Lease	(14,792)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease	0
Business Acquisition Purchase Price Allocation Intangible Liabilities Total	(14,792)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	230,924
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 Tangible Assets Other	101
Business Acquisition Purchase Price Allocation Deferred Rent Receivable	44
Business Acquisition Purchase Price Allocation Tangible Assets Total	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 Amortizable Intangible Liabilities Below Market Ground Lease	0
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option	0
Business Acquisition Purchase Price Allocation Intangible Assets Total	15,867
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Lease	(9,273)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease	0
Business Acquisition Purchase Price Allocation Intangible Liabilities Total	(9,273)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	150,851
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 Other	0
Business Acquisition Purchase Price Allocation Deferred Rent Receivable	0
Business Acquisition Purchase Price Allocation Tangible Assets Total	96,708
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Above Market Leases	89
Finite-Lived Intangible Asset, Acquired-in-Place Leases	8,222
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
Business Acquisition Purchase Price Allocation Intangible Assets Total	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 Total	(4,801)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	102,621
Texas Acquisition [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business Acquisition, Purchase Price Allocation, Land	306,563
Business Acquisition Purchase Price Allocation Tangible Assets Building	586,150
Business Acquisition Purchase Price Allocation Tangible Assets Tenant Improvements and FF and E	114,220
Business Acquisition Purchase Price Allocation Tangible Assets Other	0
Business Acquisition Purchase Price Allocation Deferred Rent Receivable	0
Business Acquisition Purchase Price Allocation Tangible Assets Total	1,006,933
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Above Market Leases	4,959
Finite-Lived Intangible Asset, Acquired-in-Place Leases	117,630
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Ground Lease	2,958
Business Acquisition Purchase Price Allocation Amortizable Intangible Assets Ground Lease Purchase Option	0
Business Acquisition Purchase Price Allocation Intangible Assets Total	125,547
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Below Market Lease	(47,170)
Business Acquisition Purchase Price Allocation Amortizable Intangible Liabilities Above Market Ground Lease	(2,508)
Business Acquisition Purchase Price Allocation Intangible Liabilities Total	(49,678)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	$ 1,082,802