Quarterly Financial Information (Unaudited) Selected Quarterly Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								8 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, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information [Line Items]
Total revenue	$ 82,605	$ 82,507	$ 85,023	$ 80,732	$ 81,746	$ 82,339	$ 77,106	$ 73,792	$ 12,650	$ 330,867	$ 314,983	$ 295,088
Operating income (loss)	22,091	27,275	17,249	12,920	23,792	26,477	20,048	23,329	(3,670)	79,535	93,646	97,941
Net income	9,209	14,271	4,413	(691)	9,731	12,505	7,588	10,308	$ (3,670)	27,202	40,132	45,637
Net income attributable to restricted shares	(96)	(71)	(216)	72	(60)	(60)	(61)	(60)		(311)	(241)	(189)
Net income attributable to unitholders in the Operating Partnership	(2,440)	(3,806)	(1,125)	166	(2,594)	(3,351)	(2,008)	(2,861)		(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
Net income attributable to common stockholders - basic and diluted (in dollars per share)	$ 0.14	$ 0.22	$ 0.07	$ (0.01)	$ 0.15	$ 0.19	$ 0.12	$ 0.16
American Assets Trust, L.P.
Quarterly Financial Information [Line Items]
Total revenue	$ 82,605	$ 82,507	$ 85,023	$ 80,732	$ 81,746	$ 82,339	$ 77,106	$ 73,792		330,867	314,983	295,088
Operating income (loss)	22,091	27,275	17,249	12,920	23,792	26,477	20,048	23,329		79,535	93,646	97,941
Net income	9,209	14,271	4,413	(691)	9,731	12,505	7,588	10,308		27,202	40,132	45,637
Net income attributable to restricted shares	(96)	(71)	(216)	72	(60)	(60)	(61)	(60)		(311)	(241)	(189)
Net income attributable to American Assets Trust, Inc. stockholders										$ 26,891	$ 39,891	$ 45,448
Net income attributable to American Assets Trust, L.P. unit holders	$ 9,113	$ 14,200	$ 4,197	$ (619)	$ 9,671	$ 12,445	$ 7,527	$ 10,248
Net income attributable to common stockholders - basic and diluted (in dollars per share)	$ 0.14	$ 0.22	$ 0.07	$ (0.01)	$ 0.15	$ 0.19	$ 0.12	$ 0.16