SCHEDULE III SCHEDULE III (Details2) - USD ($)		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017
Year Constructed		1998
Encumbrances	[1],[2]	$ 133,422,000
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at beginning of year	[3]	2,817,145,000		$ 2,578,748,000	$ 2,407,029,000
Purchases of real estate properties		135,033,000		54,537,000	51,802,000
Development of real estate properties		318,288,000		167,667,000	124,938,000
Improvements to real estate properties		37,558,000		36,921,000	28,698,000
Right-of-use asset, ground leases, net of amortization		11,997,000		0	0
Carrying amount of investments sold		(51,662,000)		(18,372,000)	(32,787,000)
Write-off of improvements		(3,793,000)		(2,356,000)	(932,000)
Balance at end of year	[3]	3,264,566,000	[1],[2]	2,817,145,000	2,578,748,000
Noncontrolling Interests		3,148,000		3,296,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at beginning of year		814,915,000		749,601,000	694,250,000
Depreciation expense		86,590,000		76,007,000	69,010,000
Accumulated depreciation on assets sold		(27,030,000)		(8,670,000)	(12,735,000)
Other		(3,336,000)		(2,023,000)	(924,000)
Balance at end of year		871,139,000	[1],[2]	814,915,000	$ 749,601,000
Estimated aggregate cost of real estate properties for federal income tax purposes		3,179,014,000
Estimated accumulated tax depreciation		$ 619,405,000
Length of Time After Project Completion When Development Cost Ceased Being Capitalized		1 year
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Land	[1],[2]	$ 588,765,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Building and Improvements	[1],[2]	1,210,776,000
Costs Capitalized Subsequent to Acquisition	[1],[2]	1,453,028,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Land, Amount	[1],[2]	591,945,000
Buildings and Improvements	[1],[2]	2,660,624,000
Secured debt, carrying amount		$ 133,422,000		189,038,000
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, carrying amount		$ 0		46,725,000
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, carrying amount	[4]	44,596,000		47,445,000
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, carrying amount		37,682,000		40,046,000
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, carrying amount		48,772,000		$ 52,115,000
Industrial Development [Member]
Encumbrances		0
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at end of year		419,999,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at end of year		1,151,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Land		140,364,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Building and Improvements		68,205,000
Costs Capitalized Subsequent to Acquisition		211,430,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Land, Amount		139,247,000
Buildings and Improvements		$ 280,752,000
Development properties [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Percentage of Occupation when property transfers from the development and value-add program to real estate properties		90.00%
Length of Time After Project Acquisition When Project Transfers to Real Estate Properties		1 year
Value-add properties [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Percentage of Occupation when property transfers from the development and value-add program to real estate properties		90.00%
Length of Time After Project Acquisition When Project Transfers to Real Estate Properties		1 year
Development and value-add properties [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Percentage of Occupation When Costs Ceased Being Capitalized for development and value-add properties		90.00%
Length of Time After Project Completion When Development Cost Ceased Being Capitalized		1 year
University Business Center 120 [Member]
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Noncontrolling Interests (in hundreths)		80.00%

[1] The estimated aggregate cost of real estate properties at December 31, 2019 for federal income tax purposes was approximately $3,179,014,000 before estimated accumulated tax depreciation of $619,405,000.  The federal income tax return for the year ended December 31, 2019, has not been filed and accordingly, this estimate is based on preliminary data.

[2] Changes in Real Estate Properties follow:                                                                                                                                                                                                                                                                                                                                                                                               Years Ended December 31, 2019   2018   2017 (In thousands) Balance at beginning of year  $ 2,817,145
  2,578,748
  2,407,029
Purchases of real estate properties  135,033
  54,537
  51,802
Development of real estate properties and value-add properties 318,288
  167,667
  124,938
Improvements to real estate properties 37,558
  36,921
  28,698
Right-of-use assets, net – ground leases 11,997
  —
  —
Carrying amount of investments sold  (51,662 )   (18,372 )   (32,787 ) Write-off of improvements  (3,793 )   (2,356 )   (932 ) Balance at end of year ⁽¹⁾  $ 3,264,566
  2,817,145
  2,578,748

(1) Includes noncontrolling interest in joint ventures of $3,148,000 and $3,296,000 at December 31, 2019 and 2018, respectively.
Changes in the accumulated depreciation on real estate properties follow:                                                                                                                                                                                                                                                                                                                                                                                     Years Ended December 31, 2019   2018   2017 (In thousands) Balance at beginning of year  $ 814,915
  749,601
  694,250
Depreciation expense  86,590
  76,007
  69,010
Accumulated depreciation on assets sold  (27,030 )   (8,670 )   (12,735 ) Other  (3,336 )   (2,023 )   (924 ) Balance at end of year  $ 871,139
  814,915
  749,601

[3] Includes noncontrolling interest in joint ventures of $3,148,000 and $3,296,000 at December 31, 2019 and 2018, respectively.

[4] During 2019, the Company executed a collateral release for World Houston 5; this property was sold during 2019 and is no longer considered to be collateral securing this loan.