SCHEDULE III (Details2) - USD ($)		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
Encumbrances	[1],[2]	$ 79,096,000
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at beginning of year	[3]	3,264,566,000		$ 2,817,145,000	$ 2,578,748,000
Purchases of real estate properties		46,240,000		135,033,000	54,537,000
Development of real estate properties		195,446,000		318,288,000	167,667,000
Improvements to real estate properties		33,522,000		37,558,000	36,921,000
Right-of-use asset, ground leases, net of amortization		(924,000)		11,997,000	0
Carrying amount of investments sold		(17,182,000)		(51,662,000)	(18,372,000)
Write-off of improvements		(2,583,000)		(3,793,000)	(2,356,000)
Balance at end of year	[3]	3,519,085,000	[1],[2]	3,264,566,000	2,817,145,000
Noncontrolling Interests		852,000		3,148,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at beginning of year		871,139,000		814,915,000	749,601,000
Depreciation expense		96,290,000		86,590,000	76,007,000
Accumulated depreciation on assets sold		(9,599,000)		(27,030,000)	(8,670,000)
Other		(2,502,000)		(3,336,000)	(2,023,000)
Balance at end of year		955,328,000	[1],[2]	871,139,000	$ 814,915,000
Estimated aggregate cost of real estate properties for federal income tax purposes		3,464,143,000
Estimated accumulated tax depreciation		688,740,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Land	[1],[2]	655,023,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Building and Improvements	[1],[2]	1,251,222,000
Costs Capitalized Subsequent to Acquisition	[1],[2]	1,601,767,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Land, Amount	[1],[2]	659,511,000
Buildings and Improvements	[1],[2]	2,848,501,000
Secured debt, carrying amount		$ 79,096,000		133,422,000
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
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]	$ 41,610,000		44,596,000
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		35,220,000		37,682,000
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		0		$ 48,772,000
Industrial Development [Member]
Encumbrances		0
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at end of year		359,588,000
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at end of year		755,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Land		156,415,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost of Building and Improvements		18,228,000
Costs Capitalized Subsequent to Acquisition		184,945,000
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Land, Amount		156,772,000
Buildings and Improvements		$ 202,816,000
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

[1] Changes in Real Estate Properties and Development and Value-Add Properties follow:                                                                                                                                                                                                                                                                                                                                                                                    Years Ended December 31,202020192018(In thousands)Balance at beginning of year $3,264,566 2,817,145 2,578,748 Purchases of real estate properties 46,240 135,033 54,537 Development of real estate properties and value-add properties195,446 318,288 167,667 Improvements to real estate properties33,522 37,558 36,921 Right-of-use assets, net – ground leases(924)11,997 — Carrying amount of investments sold (17,182)(51,662)(18,372)Write-off of improvements (2,583)(3,793)(2,356) Balance at end of year (1)  $3,519,085 3,264,566 2,817,145 
(1)Includes noncontrolling interest in joint ventures of $852,000 and $3,148,000 at December 31, 2020 and 2019, respectively.
Changes in the accumulated depreciation on real estate properties follow:                                                                                                                                                                                                                                                                                                                                                                                   Years Ended December 31,202020192018(In thousands)Balance at beginning of year $871,139 814,915 749,601 Depreciation expense 96,290 86,590 76,007 Accumulated depreciation on assets sold (9,599)(27,030)(8,670)Other (2,502)(3,336)(2,023)Balance at end of year $955,328 871,139 814,915 

[2] The estimated aggregate cost of real estate properties at December 31, 2020 for federal income tax purposes was approximately $3,464,143,000 before estimated accumulated tax depreciation of $688,740,000.  The federal income tax return for the year ended December 31, 2020, has not been filed and accordingly, this estimate is based on preliminary data.

[3] Includes noncontrolling interest in joint ventures of $852,000 and $3,148,000 at December 31, 2020 and 2019, 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.