SEGMENT REPORTING - Net Real Estate and Secured Note Payable Balances (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net Real Estate		$ 2,498,564	$ 2,523,475
Secured Notes Payable	[1]	111,000	162,003
Debt Issuance Costs, Net		0	1,256
American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Net Real Estate		2,498,564	2,523,475
Secured notes payable, net | American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Debt Issuance Costs, Net		87	124
Office
Segment Reporting Information [Line Items]
Net Real Estate		1,313,885	1,317,854
Secured Notes Payable	[1]	111,000	127,768
Retail
Segment Reporting Information [Line Items]
Net Real Estate		612,718	624,912
Secured Notes Payable	[1]	0	34,235
Multifamily
Segment Reporting Information [Line Items]
Net Real Estate		392,506	401,152
Mixed-use
Segment Reporting Information [Line Items]
Net Real Estate		$ 179,455	$ 179,557

[1] Excludes debt issuance costs of $0.1 million and $0.1 million for each of the periods ended September 30, 2020 and December 31, 2019, respectively.