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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
NUMERATOR
Net income from operations									$ 60,188	$ 27,202	$ 40,132
Less: Net income attributable to restricted shares	$ (104)	$ (92)	$ (92)	$ (93)	$ (96)	$ (71)	$ (216)	$ 72	(381)	(311)	(241)
Less: Income attributable to unitholders in the Operating Partnership									14,089	7,205	10,814
NET INCOME ATTRIBUTABLE TO AMERICAN ASSETS TRUST, INC. STOCKHOLDERS	$ 12,845	$ 12,862	$ 8,916	$ 11,095	$ 6,673	$ 10,394	$ 3,072	$ (453)	45,718	19,686	29,077
Income attributable to American Assets Trust, Inc. common stockholders—basic									45,718	19,686	29,077
Plus: Income attributable to unitholders in the Operating Partnership									(14,089)	(7,205)	(10,814)
Net income attributable to common stockholders-diluted									$ 59,807	$ 26,891	$ 39,891
DENOMINATOR
Weighted average shares of common stock outstanding-basic (in shares)									54,110,949	46,950,812	46,715,520
Effect of dilutive securities-conversion of Operating Partnership units (in shares)									16,675,183	17,185,747	17,371,730
Weighted average common shares outstanding - diluted (in shares)									70,786,132	64,136,559	64,087,250
Earnings (loss) per common share-basic
Basic net income attributable to common stockholders per share (in dollars per share)									$ 0.84	$ 0.42	$ 0.62
Earnings (loss) per common share-diluted
Diluted net income attributable to common stockholders per share (in dollars per share)									$ 0.84	$ 0.42	$ 0.62