Investment in Office and Parking Properties - 2015 Dispositions (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 11, 2015	Sep. 03, 2015	Sep. 01, 2015	Jul. 31, 2015	Jul. 16, 2015	Jul. 07, 2015	Jun. 12, 2015	Jun. 05, 2015	May. 13, 2015	May. 08, 2015	Apr. 08, 2015	Feb. 04, 2015	Jan. 15, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) in continuing operations	$ 0	$ 0	$ 0	$ 10,463
Impairment loss on real estate	0	$ 0	5,400	$ 0
Fair Value, Inputs, Level 2 | Greens Parkway
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment loss on real estate			4,400
Raymond James Tower | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price																	$ 19,300
Buyer credits																	$ 8,900
Gain (loss) in continuing operations			(117)
Honeywell Building | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price																$ 28,000
Gain (loss) in continuing operations			14,300
Two Ravinia Drive | Office Building | Fund II
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price															$ 78,000
Gain (loss) in continuing operations			29,000
Two Ravinia Drive | Office Building | Parent Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) in continuing operations			8,700
400 North Belt | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price														$ 10,200
Gain (loss) in continuing operations			(1,200)
Peachtree Dunwoody | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price													$ 53,900
Gain (loss) in continuing operations			14,300
Hillsboro Center I-IV and V | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price												$ 22,000
Gain (loss) in continuing operations			2,400
Riverplace South | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price											$ 9,000
Gain (loss) in continuing operations			466
Westshore Corporate Center and Cypress Center I-III | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price										$ 66,000
Gain (loss) in continuing operations			19,200
245 Riverside | Office Building | Fund II
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price									$ 25,100
Gain (loss) in continuing operations			7,200
245 Riverside | Office Building | Parent Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) in continuing operations			2,200
550 Greens Parkway | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price								$ 2,300
Gain (loss) in continuing operations			38
Comerica Bank Building | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price							$ 31,400
Gain (loss) in continuing operations			13,000
Squaw Peak I & II | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price						$ 51,300
Gain (loss) in continuing operations			13,300
One Commerce Green | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price					$ 47,500
Buyer credits					$ 23,500
Gain (loss) in continuing operations			(5,200)
City Centre | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross sale price	$ 6,200		6,200
Gain (loss) in continuing operations			(66)
City Centre | Office Building | Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment loss on real estate			$ 1,000