Quarterly Financial Information (Unaudited) Selected Quarterly Information (Details) - USD ($) $ / shares in Units, $ in Thousands	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, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information [Line Items]
Total revenue	$ 81,347	$ 84,374	$ 82,109	$ 96,743	$ 98,947	$ 98,362	$ 84,113	$ 85,319	$ 344,573	$ 366,741	$ 330,867
Operating income (loss)	16,415	20,323	22,995	28,848	29,993	30,384	24,487	28,821	88,581	113,685	79,535
Net income	3,788	6,490	9,826	15,484	16,485	16,519	11,941	15,243	35,588	60,188	27,202
Net income attributable to restricted shares	(123)	(87)	(69)	(104)	(104)	(92)	(92)	(93)	(383)	(381)	(311)
Net income attributable to unitholders in the Operating Partnership	(767)	(1,365)	(2,101)	(3,312)	(3,536)	(3,565)	(2,933)	(4,055)	(7,545)	(14,089)	(7,205)
Net income (loss) attributable to American Assets Trust, Inc. stockholders	$ 2,898	$ 5,038	$ 7,656	$ 12,068	$ 12,845	$ 12,862	$ 8,916	$ 11,095	27,660	45,718	19,686
Net income attributable to common stockholders - basic and diluted (in dollars per share)	$ 0.05	$ 0.08	$ 0.13	$ 0.20	$ 0.22	$ 0.22	$ 0.18	$ 0.24
American Assets Trust, L.P.
Quarterly Financial Information [Line Items]
Total revenue	$ 81,347	$ 84,374	$ 82,109	$ 96,743	$ 98,947	$ 98,362	$ 84,113	$ 85,319	344,573	366,741	330,867
Operating income (loss)	16,415	20,323	22,995	28,848	29,993	30,384	24,487	28,821	88,581	113,685	79,535
Net income	3,788	6,490	9,826	15,484	16,485	16,519	11,941	15,243	35,588	60,188	27,202
Net income attributable to restricted shares	(123)	(87)	(69)	(104)	(104)	(92)	(92)	(93)	(383)	(381)	(311)
Net income (loss) attributable to American Assets Trust, Inc. stockholders									$ 35,205	$ 59,807	$ 26,891
Net income attributable to American Assets Trust, L.P. unit holders	$ 3,665	$ 6,403	$ 9,757	$ 15,380	$ 16,381	$ 16,427	$ 11,849	$ 15,150
Net income attributable to common stockholders - basic and diluted (in dollars per share)	$ 0.05	$ 0.08	$ 0.13	$ 0.20	$ 0.22	$ 0.22	$ 0.18	$ 0.24