Equity of American Assets Trust, Inc. Computation of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
NUMERATOR
Net income from operations									$ 35,588	$ 60,188	$ 27,202
Less: Net income attributable to restricted shares	$ (123)	$ (87)	$ (69)	$ (104)	$ (104)	$ (92)	$ (92)	$ (93)	(383)	(381)	(311)
Less: Income attributable to unitholders in the Operating Partnership									7,545	14,089	7,205
NET INCOME ATTRIBUTABLE TO AMERICAN ASSETS TRUST, INC. STOCKHOLDERS	$ 2,898	$ 5,038	$ 7,656	$ 12,068	$ 12,845	$ 12,862	$ 8,916	$ 11,095	27,660	45,718	19,686
Income attributable to American Assets Trust, Inc. common stockholders—basic									27,660	45,718	19,686
Plus: Income attributable to unitholders in the Operating Partnership									(7,545)	(14,089)	(7,205)
Net income attributable to common stockholders-diluted									$ 35,205	$ 59,807	$ 26,891
DENOMINATOR
Weighted average shares of common stock outstanding-basic (in shares)									59,806,309	54,110,949	46,950,812
Effect of dilutive securities-conversion of Operating Partnership units (in shares)									16,313,454	16,675,183	17,185,747
Weighted average common shares outstanding - diluted (in shares)									76,119,763	70,786,132	64,136,559
Earnings (loss) per common share-basic
Basic net income attributable to common stockholders per share (in dollars per share)									$ 0.46	$ 0.84	$ 0.42
Earnings (loss) per common share-diluted
Diluted net income attributable to common stockholders per share (in dollars per share)									$ 0.46	$ 0.84	$ 0.42