SEGMENT REPORTING - Net Real Estate and Secured Note Payable Balances (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Net Real Estate		$ 2,680,957	$ 2,492,734
Secured Notes Payable	[1]	111,000	111,000
Debt issuance costs, net of accumulated amortization of $1,527 and $0, respectively		543	0
American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Net Real Estate		2,680,957	2,492,734
Secured notes payable, net | American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Debt issuance costs, net of accumulated amortization of $1,527 and $0, respectively		45	77
Office
Segment Reporting Information [Line Items]
Net Real Estate		1,528,715	1,317,107
Secured Notes Payable	[1]	111,000	111,000
Retail
Segment Reporting Information [Line Items]
Net Real Estate		594,491	607,918
Multifamily
Segment Reporting Information [Line Items]
Net Real Estate		383,095	389,804
Mixed-use
Segment Reporting Information [Line Items]
Net Real Estate		$ 174,656	$ 177,905

[1] Excludes debt issuance costs of $0.0 million and $0.1 million for each of the periods ended September 30, 2021 and December 31, 2020, respectively.