Equity of American Assets Trust, Inc. Dividends Declared and Paid on Shares of Common Stock and Noncontrolling Common Units (Details) - $ / shares	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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Dividends period	Fourth Quarter 2020	Third Quarter 2020	Second Quarter 2020	First Quarter 2020	Fourth Quarter 2019	Third Quarter 2019	Second Quarter 2019	First Quarter 2019	Fourth Quarter 2018	Third Quarter 2018	Second Quarter 2018	First Quarter 2018
Amount per Share/Unit (in USD per unit)	$ 0.25	$ 0.25	$ 0.20	$ 0.30	$ 0.30	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.27	$ 0.27	$ 0.27	$ 1.00	$ 1.14	$ 1.09
Date dividend to be paid	Dec. 24, 2020	Sep. 24, 2020	Jun. 25, 2020	Mar. 26, 2020	Dec. 26, 2019	Sep. 26, 2019	Jun. 27, 2019	Mar. 28, 2019	Dec. 27, 2018	Sep. 27, 2018	Jun. 28, 2018	Mar. 19, 2018