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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Dividends period	Fourth Quarter 2017	Third Quarter 2017	Second Quarter 2017	First Quarter 2017	Fourth Quarter 2016	Third Quarter 2016	Second Quarter 2016	First Quarter 2016	Fourth Quarter 2015	Third Quarter 2015	Second Quarter 2015	First Quarter 2015
Amount per Share/Unit (in USD per unit)	$ 0.27	$ 0.26	$ 0.26	$ 0.26	$ 0.26	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.2325	$ 0.2325	$ 0.2325	$ 1.05	$ 1.01	$ 0.95
Date dividend to be paid	Dec. 21, 2017	Sep. 28, 2017	Jun. 29, 2017	Mar. 30, 2017	Dec. 22, 2016	Sep. 29, 2016	Jun. 24, 2016	Mar. 25, 2016	Dec. 23, 2015	Sep. 25, 2015	Jun. 26, 2015	Mar. 27, 2015