SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) employee property room segment	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of employees | employee	228
Number of office, retail, multifamily and mixed-use operating properties | property	31
Properties held for development | property	3
Allowance for doubtful accounts	$ 1.4	$ 2.1
Total bad debt expense	1.5	2.3	$ 2.3
Real estate depreciation expense	106.3	108.1	102.0
Cash balance at banks, excess of FDIC insured limit	64.1	39.9
Restricted cash	0.0	0.0
Incremental stock compensation cost from modification	$ 0.7	1.4	2.6
Number of operating segments | segment	4
Rooms in mixed-use segment all-suite hotel | room	369
Earnest S. Rady
Summary Of Significant Accounting Policies [Line Items]
Properties owned through controlling interest | property	17
Properties owned through noncontrolling interest | property	4
American Assets Trust, L.P. | Unsecured Debt | Term Loan B & Term Loan C
Summary Of Significant Accounting Policies [Line Items]
Face amount of debt	$ 225.0	150.0
Money Market Funds
Summary Of Significant Accounting Policies [Line Items]
Cash balance at banks, excess of FDIC insured limit	$ 12.3	0.5
Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of taxable income required to distribute to qualify as real estate investment trust (REIT)	90.00%
Building And Improvement | Minimum
Summary Of Significant Accounting Policies [Line Items]
Real Estate, estimated useful lives	30 years
Building And Improvement | Maximum
Summary Of Significant Accounting Policies [Line Items]
Real Estate, estimated useful lives	40 years
Furniture And Equipment | Minimum
Summary Of Significant Accounting Policies [Line Items]
Real Estate, estimated useful lives	3 years
Furniture And Equipment | Maximum
Summary Of Significant Accounting Policies [Line Items]
Real Estate, estimated useful lives	15 years
Other Income
Summary Of Significant Accounting Policies [Line Items]
General excise tax recognized, gross	$ 3.7	$ 3.4	$ 2.5