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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Dividends period	Fourth Quarter 2018	Third Quarter 2018	Second Quarter 2018	First Quarter 2018	Fourth Quarter 2017	Third Quarter 2017	Second Quarter 2017	First Quarter 2017	Fourth Quarter 2016	Third Quarter 2016	Second Quarter 2016	First Quarter 2016
Amount per Share/Unit (in USD per unit)	$ 0.28	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.26	$ 0.26	$ 0.26	$ 0.26	$ 0.25	$ 0.25	$ 0.25	$ 1.09	$ 1.05	$ 1.01
Date dividend to be paid	Dec. 27, 2018	Sep. 27, 2018	Jun. 28, 2018	Mar. 19, 2018	Dec. 21, 2017	Sep. 28, 2017	Jun. 29, 2017	Mar. 30, 2017	Dec. 22, 2016	Sep. 29, 2016	Jun. 24, 2016	Mar. 25, 2016