SCHEDULE III SCHEDULE III (Details2) - USD ($)		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Year Constructed		1998
Encumbrances		$ 189,038,000
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at beginning of year	[1]	2,578,748,000		$ 2,407,029,000	$ 2,219,465,000
Purchases of real estate properties		54,537,000		51,802,000	22,228,000
Development of real estate properties		167,667,000		124,938,000	203,765,000
Improvements to real estate properties		36,921,000		28,698,000	23,188,000
Carrying amount of investments sold		(18,372,000)		(32,787,000)	(61,121,000)
Write-off of improvements		(2,356,000)		(932,000)	(496,000)
Balance at end of year	[1]	2,817,145,000	[2],[3]	2,578,748,000	2,407,029,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at beginning of year		749,601,000		694,250,000	657,454,000
Depreciation expense		76,007,000		69,010,000	63,793,000
Accumulated depreciation on assets sold		(8,670,000)		(12,735,000)	(26,501,000)
Other		(2,023,000)		(924,000)	(496,000)
Balance at end of year		814,915,000	[2],[3]	$ 749,601,000	$ 694,250,000
Estimated aggregate cost of real estate properties for federal income tax purposes		2,771,697,000
Estimated accumulated tax depreciation		$ 573,441,000
Percentage of Occupation When Development Cost Ceased Being Capitalized		90.00%		80.00%
Length of Time After Project Completion When Development Cost Ceased Being Capitalized		1 year		1 year
Secured debt		$ 188,461,000		$ 199,512,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Land	[2],[3]	484,279,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Building and Improvements	[2],[3]	1,087,145,000
Costs Capitalized Subsequent to Acquisition	[2],[3]	1,245,721,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Land, Amount	[2],[3]	487,477,000
Buildings and Improvements	[2],[3]	$ 2,329,668,000
University Business Center [Member]
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Noncontrolling Interests (in hundreths)		20.00%		20.00%
Noncontrolling Interests		$ 3,296,000		$ 3,217,000
Buildings [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Estimated useful life (in years)		40 years
Minimum [Member] | Improvements and Personal Property [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Estimated useful life (in years)		3 years
Maximum [Member] | Improvements and Personal Property [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Estimated useful life (in years)		15 years
Dominguez, Industry I and III, Kingsview, Shaw, Walnut and Washington [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Secured debt		$ 46,725,000		49,580,000
Colorado Crossing, Interstate I-III, Rojas, Steele Creek 1 & 2, Venture and World Houston 3-9
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Secured debt	[4]	47,445,000		50,161,000
Arion 18, Beltway VI & VII, Commerce Park II & III, Concord Dist Ctr, Interstate Dist Ctr V, VI & VII, Lakeview Business Ctr, Ridge Creek Distribution Ctr II, Southridge IV & V and World Houston 32 [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Secured debt		40,046,000		42,315,000
40th Avenue, Beltway Crossing V, Centennial Park, Executive Airport, Interchange Park I, Ocean View, Wetmore 5-8 and World Houston 26, 28, 29 and 30 [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Secured debt		52,115,000		$ 55,317,000
Industrial Development [Member]
Encumbrances	[5]	0
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at end of year	[5]	263,664,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at end of year	[5]	184,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Land	[5]	105,736,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Building and Improvements	[5]	9,211,000
Costs Capitalized Subsequent to Acquisition	[5]	148,717,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Land, Amount	[5]	106,793,000
Buildings and Improvements	[5]	$ 156,871,000

[1] Includes 20% noncontrolling interest in University Business Center of $3,296,000 and $3,217,000 at December 31, 2018 and 2017, respectively.

[2] Changes in Real Estate Properties follow: Years Ended December 31,2018 2017 2016(In thousands)Balance at beginning of year $2,578,748 2,407,029 2,219,465Purchases of real estate properties 54,537 51,802 22,228Development of real estate properties and value-add properties167,667 124,938 203,765Improvements to real estate properties36,921 28,698 23,188Carrying amount of investments sold (18,372) (32,787) (61,121)Write-off of improvements (2,356) (932) (496)Balance at end of year (1) $2,817,145 2,578,748 2,407,029(1)Includes 20% noncontrolling interest in University Business Center of $3,296,000 and $3,217,000 at December 31, 2018 and 2017, respectively.Changes in the accumulated depreciation on real estate properties follow: Years Ended December 31,2018 2017 2016(In thousands)Balance at beginning of year $749,601 694,250 657,454Depreciation expense 76,007 69,010 63,793Accumulated depreciation on assets sold (8,670) (12,735) (26,501)Other (2,023) (924) (496)Balance at end of year $814,915 749,601 694,250

[3] The estimated aggregate cost of real estate properties at December 31, 2018 for federal income tax purposes was approximately $2,771,697,000 before estimated accumulated tax depreciation of $573,441,000. The federal income tax return for the year ended December 31, 2018, has not been filed and accordingly, this estimate is based on preliminary data.

[4] Subsequent to December 31, 2018, the Company executed a collateral release for World Houston 5; this property is no longer considered to be collateral securing this loan.

[5] Effective January 1, 2018, the Company began transferring development projects from Development and value-add properties to Real estate properties at the earlier of 90% occupancy or one year after completion of the shell construction. Prior to 2018, the Company transferred development projects to Real estate properties the earlier of 80% occupancy or one year after completion of the shell construction.