Investment in Office Properties - Dispositions (Details) - USD ($)				12 Months Ended
	Sep. 04, 2014	Jan. 31, 2014	Jan. 14, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	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	Dec. 29, 2014	Dec. 23, 2014	Oct. 06, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of real estate from discontinued operations				$ 0	$ 10,463,000	$ 32,493,000
Impairment loss on real estate				5,400,000	11,700,000	0
Gain on sale of real estate				110,732,000	76,378,000	$ 0
Raymond James Tower | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price																				$ 19,300,000
Buyer credits																				$ 8,900,000
Gain (loss) on sale of real estate from discontinued operations				(117,000)
Honeywell Building | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price																			$ 28,000,000
Gain (loss) on sale of real estate from discontinued operations				14,300,000
Two Ravinia Drive | Office Building | Parent Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of real estate from discontinued operations				8,700,000
Two Ravinia Drive | Office Building | Fund II
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price																		$ 78,000,000
Gain (loss) on sale of real estate from discontinued operations				29,000,000
400 North Belt | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price																	$ 10,200,000
Gain (loss) on sale of real estate from discontinued operations				(1,200,000)
Peachtree Dunwoody | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price																$ 53,900,000
Gain (loss) on sale of real estate from discontinued operations				14,500,000
Hillsboro I-IV and V | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price															$ 22,000,000
Gain (loss) on sale of real estate from discontinued operations				2,400,000
Riverplace South | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price														$ 9,000,000
Gain (loss) on sale of real estate from discontinued operations				466,000
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]
Net Sales Price													$ 66,000,000
Gain (loss) on sale of real estate from discontinued operations				19,200,000
245 Riverside | Office Building | Parent Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of real estate from discontinued operations				2,200,000
245 Riverside | Office Building | Fund II
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price												$ 25,100,000
Gain (loss) on sale of real estate from discontinued operations				7,200,000
550 Greens Parkway | Fair Value, Inputs, Level 2
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment loss on real estate				4,400,000
550 Greens Parkway | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price											$ 2,300,000
Gain (loss) on sale of real estate from discontinued operations				37,000
Comerica Bank Building | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price										$ 31,400,000
Gain (loss) on sale of real estate from discontinued operations				13,000,000
Squaw Peak I & II | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price									$ 51,300,000
Gain (loss) on sale of real estate from discontinued operations				13,200,000
One Commerce Green | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price								$ 47,500,000
Buyer credits								$ 23,500,000
Gain (loss) on sale of real estate from discontinued operations				(5,200,000)
City Centre | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price							$ 6,200,000
Gain (loss) on sale of real estate from discontinued operations				(108,000)
City Centre | Office Building | Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment loss on real estate				1,000,000
Millenia Park One | Office Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price																						$ 28,200,000
Gain (loss) on sale of real estate from discontinued operations				$ 3,500,000
Woodbranch Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price					14,424,000
Gain (loss) on sale of real estate from discontinued operations					9,974,000
Impairment loss on real estate					0
Gross sales price of assets sold			$ 15,000,000
Proceeds from sale of real estate			$ 13,900,000
Gain on sale of properties					10,000,000
Mesa Corporate Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price					12,257,000
Gain (loss) on sale of real estate from discontinued operations					489,000
Impairment loss on real estate					5,600,000
Gross sales price of assets sold		$ 13,200,000
Proceeds from sale of real estate		$ 12,100,000
Gain on sale of properties					489,000
Schlumberger Building | Houston, TX
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price	$ 17,000,000
Proceeds from sale of real estate	$ 16,200,000
Gain on sale of real estate					6,700,000
Tempe Town Lake | Parent Company | Tempe, Arizona
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of real estate					221,700
Tempe Town Lake | Fund II | Tempe, Arizona
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price																							$ 2,000,000
Gain on sale of real estate					739,000
525 North Tryon | Charlotte, North Carolina
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Sales Price																					$ 60,000,000
Gain on sale of real estate					$ 16,100,000