SEGMENT REPORTING - Net Real Estate and Secured Note Payable Balances (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net Real Estate		$ 2,035,567	$ 2,076,707
Secured Notes Payable	[1]	205,047	279,889
American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Net Real Estate		2,035,567	2,076,707
Secured notes payable, net | American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Debt issuance costs, net		200	300
Total Retail
Segment Reporting Information [Line Items]
Net Real Estate		629,484	658,654
Secured Notes Payable	[1]	35,196	35,737
Total Office
Segment Reporting Information [Line Items]
Net Real Estate		813,623	813,121
Secured Notes Payable	[1]	169,851	170,408
Total Multifamily
Segment Reporting Information [Line Items]
Net Real Estate		414,983	424,044
Secured Notes Payable	[1]	0	73,744
Total Mixed-Use
Segment Reporting Information [Line Items]
Net Real Estate		177,477	180,888
Secured Notes Payable	[1]	$ 0	$ 0

[1] Excludes debt issuance costs of $0.2 million and $0.3 million for each of the periods ending September 30, 2018 and December 31, 2017, respectively.