SCHEDULE III SCHEDULE III (Details2) - USD ($)		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at beginning of year	[1]	$ 2,406,981,000		$ 2,219,448,000	$ 2,074,946,000
Purchases of real estate properties		51,802,000		22,228,000	28,648,000
Development of real estate properties		124,938,000		203,765,000	95,032,000
Improvements to real estate properties		27,471,000		23,157,000	25,778,000
Carrying amount of investments sold		(32,787,000)		(61,121,000)	(4,750,000)
Write-off of improvements		(932,000)		(496,000)	(206,000)
Balance at end of year	[1]	2,577,473,000	[2],[3]	2,406,981,000	2,219,448,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at beginning of year		694,250,000		657,454,000	600,526,000
Depreciation expense		69,010,000		63,793,000	59,882,000
Accumulated depreciation on assets sold		(12,735,000)		(26,501,000)	(2,748,000)
Other		(924,000)		(496,000)	(206,000)
Balance at end of year		749,601,000	[2],[3]	694,250,000	$ 657,454,000
Estimated aggregate cost of real estate properties for federal income tax purposes		2,536,820,000
Estimated accumulated tax depreciation		$ 518,257,000
Percentage of Occupation When Development Cost Ceased Being Capitalized		80.00%
Length of Time After Project Completion When Development Cost Ceased Being Capitalized		1 year
Secured debt		$ 199,512,000		257,505,000
University Business Center [Member]
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Noncontrolling Interests (in hundreths)		20.00%
Noncontrolling Interests		$ 3,217,000		6,853,000
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		$ 49,580,000		52,231,000
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		50,161,000		52,752,000
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		42,315,000		44,493,000
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		$ 55,317,000		$ 58,380,000
Development transfer to real estate properties [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Percentage of Occupation When Development Cost Ceased Being Capitalized		90.00%
Length of Time After Project Completion When Development Cost Ceased Being Capitalized		1 year

[1] Includes 20% noncontrolling interest in University Business Center of $3,217,000 and $6,853,000 at December 31, 2017 and 2016, respectively.

[2] Changes in Real Estate Properties follow: Years Ended December 31,2017 2016 2015(In thousands)Balance at beginning of year $2,406,981 2,219,448 2,074,946Purchases of real estate properties 51,802 22,228 28,648Development of real estate properties124,938 203,765 95,032Improvements to real estate properties27,471 23,157 25,778Carrying amount of investments sold (32,787) (61,121) (4,750)Write-off of improvements (932) (496) (206)Balance at end of year (1) $2,577,473 2,406,981 2,219,448(1)Includes 20% noncontrolling interest in University Business Center of $3,217,000 and $6,853,000 at December 31, 2017 and 2016, respectively.Changes in the accumulated depreciation on real estate properties follow: Years Ended December 31,2017 2016 2015(In thousands)Balance at beginning of year $694,250 657,454 600,526Depreciation expense 69,010 63,793 59,882Accumulated depreciation on assets sold (12,735) (26,501) (2,748)Other (924) (496) (206)Balance at end of year $749,601 694,250 657,454

[3] The estimated aggregate cost of real estate properties at December 31, 2017 for federal income tax purposes was approximately $2,536,820,000 before estimated accumulated tax depreciation of $518,257,000. The federal income tax return for the year ended December 31, 2017, has not been filed and accordingly, this estimate is based on preliminary data.