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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
NUMERATOR
Net income from operations									$ 27,202	$ 40,132	$ 45,637
Less: Net income attributable to restricted shares	$ (96)	$ (71)	$ (216)	$ 72	$ (60)	$ (60)	$ (61)	$ (60)	(311)	(241)	(189)
Less: Income from operations attributable to unitholders in the Operating Partnership									7,205	10,814	12,863
NET INCOME ATTRIBUTABLE TO AMERICAN ASSETS TRUST, INC. STOCKHOLDERS	$ 6,673	$ 10,394	$ 3,072	$ (453)	$ 7,077	$ 9,094	$ 5,519	$ 7,387	19,686	29,077	32,585
Income from operations attributable to American Assets Trust, Inc. common stockholders—basic									19,686	29,077	32,585
Plus: Income from operations attributable to unitholders in the Operating Partnership									(7,205)	(10,814)	(12,863)
Net income attributable to common stockholders-diluted									$ 26,891	$ 39,891	$ 45,448
DENOMINATOR
Weighted average shares of common stock outstanding-basic (in shares)									46,950,812	46,715,520	45,332,471
Effect of dilutive securities-conversion of Operating Partnership units (in shares)									17,185,747	17,371,730	17,895,688
Weighted average common shares outstanding - diluted (in shares)									64,136,559	64,087,250	63,228,159
Earnings (loss) per common share-basic
Basic net income attributable to common stockholders per share (in dollars per share)									$ 0.42	$ 0.62	$ 0.72
Earnings (loss) per common share-diluted
Diluted net income attributable to common stockholders per share (in dollars per share)									$ 0.42	$ 0.62	$ 0.72