DEBT - Narrative (Details)				3 Months Ended		9 Months Ended
	Jan. 26, 2021 USD ($)	Jan. 09, 2019 Extension_Option	Jan. 09, 2018 USD ($) Extension_Option	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Unsecured line of credit, net				$ 0		$ 0		$ 99,151,000
Repayments of Unsecured Debt						155,375,000	$ 0
Early extinguishment of debt				0	$ 0	(4,271,000)	0
Repayments of Lines of Credit						100,000,000	0
Unsecured term loans, net | Term Loan B | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on federal funds rate			0.50%
Unsecured term loans, net | Term Loan B | Eurodollar
Debt Instrument [Line Items]
Basis spread on Eurodollar rate			1.00%
Unsecured term loans, net | Term Loan C | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on federal funds rate			0.50%
Unsecured term loans, net | Term Loan C | Eurodollar
Debt Instrument [Line Items]
Basis spread on Eurodollar rate			1.00%
Unsecured term loans, net | Term Loan A | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on federal funds rate		0.50%
Unsecured term loans, net | Term Loan A | Eurodollar
Debt Instrument [Line Items]
Basis spread on Eurodollar rate		1.00%
Unsecured term loans, net | Second Amended and Restated Credit Facility | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Description of variable rate basis			LIBOR
Unsecured line of credit, net
Debt Instrument [Line Items]
Minimum fixed charge coverage ratio covenant threshold			1.50
Minimum unsecured leverage ratio			175.00%
Maximum leverage ratio of revolving credit facility			60.00%
Maximum secured leverage ratio on revolving credit facility			40.00%
Maximum unsecured leverage ratio			60.00%
Maximum recourse indebtedness of total asset value			15.00%
Unsecured line of credit, net | Second Amended and Restated Credit Facility | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate			1.00%
Unsecured line of credit, net | Second Amended and Restated Credit Facility | Base Rate
Debt Instrument [Line Items]
Basis spread on federal funds rate			0.50%
American Assets Trust, L.P.
Debt Instrument [Line Items]
Unsecured line of credit, net				0		0		99,151,000
Repayments of Unsecured Debt						155,375,000	0
Early extinguishment of debt				0	$ 0	(4,271,000)	0
Repayments of Lines of Credit						100,000,000	$ 0
American Assets Trust, L.P. | Term Loan B
Debt Instrument [Line Items]
Term of debt instrument			7 years
American Assets Trust, L.P. | Term Loan B | Minimum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate			0.70%
American Assets Trust, L.P. | Term Loan B | Maximum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate			1.35%
American Assets Trust, L.P. | Term Loan C
Debt Instrument [Line Items]
Term of debt instrument			7 years
American Assets Trust, L.P. | Term Loan C | Minimum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate			0.70%
American Assets Trust, L.P. | Term Loan C | Maximum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate			1.35%
American Assets Trust, L.P. | Term Loan A | Minimum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate		0.20%
American Assets Trust, L.P. | Term Loan A | Maximum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate		0.70%
American Assets Trust, L.P. | Second Amended and Restated Credit Facility
Debt Instrument [Line Items]
Revolving credit facility borrowing limit, maximum borrowing capacity			$ 450,000,000
Additional borrowing capacity			$ 250,000,000
American Assets Trust, L.P. | 3.375% Senior Unsecured Notes
Debt Instrument [Line Items]
Stated Interest Rate	3.375%
Proceeds from Debt, Net of Issuance Costs	$ 489,700,000
Debt Securities, Held-to-Maturity, Weighted Average Yield, Maturity, after Year 10	3.502%
Note Offering Percent	0.98935
American Assets Trust, L.P. | Unsecured term loans, net
Debt Instrument [Line Items]
Long-term Debt, Principal Balance				1,550,000,000		1,550,000,000		1,200,000,000
American Assets Trust, L.P. | Unsecured term loans, net | Senior Guaranteed Notes, Series G
Debt Instrument [Line Items]
Long-term Debt, Principal Balance				$ 150,000,000		$ 150,000,000		150,000,000
Stated Interest Rate				3.91%		3.91%
Stated Maturity Date						Jul. 30, 2030
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan B
Debt Instrument [Line Items]
Long-term Debt, Principal Balance				$ 100,000,000		$ 100,000,000		100,000,000
Stated Maturity Date						Mar. 01, 2023
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan B | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Description of variable rate basis			LIBOR
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan B | Base Rate
Debt Instrument [Line Items]
Minimum base rate			0.00%
Prime rate			prime rate
Federal funds rate			federal funds rate
Eurodollar rate			Eurodollar
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan B | Minimum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate			1.20%
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan B | Maximum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate			1.70%
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan C
Debt Instrument [Line Items]
Long-term Debt, Principal Balance				50,000,000		$ 50,000,000		50,000,000
Stated Maturity Date						Mar. 01, 2023
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan C | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Description of variable rate basis			LIBOR
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan C | Base Rate
Debt Instrument [Line Items]
Minimum base rate			0.00%
Prime rate			prime rate
Federal funds rate			federal funds rate
Eurodollar rate			Eurodollar
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan C | Minimum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate			1.20%
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan C | Maximum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate			1.70%
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan A
Debt Instrument [Line Items]
Long-term Debt, Principal Balance				100,000,000		$ 100,000,000		100,000,000
Stated Maturity Date						Jan. 09, 2022
Debt extension options | Extension_Option		3
Extension term		1 year
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan A | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Description of variable rate basis		LIBOR
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan A | Base Rate
Debt Instrument [Line Items]
Prime rate		prime rate
Federal funds rate		federal funds rate
Eurodollar rate		Eurodollar
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan A | Minimum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate		1.20%
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan A | Maximum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate		1.70%
American Assets Trust, L.P. | Unsecured term loans, net | Term Loan B & Term Loan C
Debt Instrument [Line Items]
Face amount of debt			$ 150,000,000
American Assets Trust, L.P. | Unsecured term loans, net | Second Amended and Restated Credit Facility
Debt Instrument [Line Items]
Revolving credit facility borrowing limit, maximum borrowing capacity			$ 100,000,000
American Assets Trust, L.P. | Unsecured term loans, net | Second Amended and Restated Credit Facility | Minimum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate			1.30%
American Assets Trust, L.P. | Unsecured term loans, net | Second Amended and Restated Credit Facility | Minimum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate			0.30%
American Assets Trust, L.P. | Unsecured term loans, net | Second Amended and Restated Credit Facility | Maximum | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate			1.90%
American Assets Trust, L.P. | Unsecured term loans, net | Second Amended and Restated Credit Facility | Maximum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate			0.90%
American Assets Trust, L.P. | Unsecured term loans, net | Senior Guaranteed Notes, Series A
Debt Instrument [Line Items]
Long-term Debt, Principal Balance				$ 0		$ 0		150,000,000
Stated Interest Rate				4.04%		4.04%
Stated Maturity Date						Oct. 31, 2021
Repayments of Unsecured Debt	$ 150,000,000
Early extinguishment of debt	3,900,000
American Assets Trust, L.P. | Unsecured term loans, net | 3.375% Senior Unsecured Notes
Debt Instrument [Line Items]
Long-term Debt, Principal Balance				$ 500,000,000		$ 500,000,000		$ 0
Stated Interest Rate				3.38%		3.38%
Stated Maturity Date						Feb. 01, 2031
Face amount of debt	500,000,000
American Assets Trust, L.P. | Unsecured line of credit, net | Second Amended and Restated Credit Facility
Debt Instrument [Line Items]
Debt issuance costs, net, revolving credit facility				$ (500,000)		$ (500,000)
Weighted average interest rate, revolving credit facility						1.19%
Repayments of Lines of Credit	$ 100,000,000
American Assets Trust, L.P. | Unsecured line of credit, net | Second Amended and Restated Credit Facility | Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility borrowing limit, maximum borrowing capacity			$ 350,000,000
Debt extension options | Extension_Option			2
American Assets Trust, L.P. | Unsecured line of credit, net | Second Amended and Restated Credit Facility | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility
Debt Instrument [Line Items]
Description of variable rate basis			LIBOR
American Assets Trust, L.P. | Unsecured line of credit, net | Second Amended and Restated Credit Facility | Base Rate
Debt Instrument [Line Items]
Prime rate			prime rate
Federal funds rate			federal funds rate
American Assets Trust, L.P. | Unsecured line of credit, net | Second Amended and Restated Credit Facility | Minimum | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate			1.05%
American Assets Trust, L.P. | Unsecured line of credit, net | Second Amended and Restated Credit Facility | Minimum | Base Rate | Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate			0.10%
American Assets Trust, L.P. | Unsecured line of credit, net | Second Amended and Restated Credit Facility | Maximum | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate			1.50%
American Assets Trust, L.P. | Unsecured line of credit, net | Second Amended and Restated Credit Facility | Maximum | Base Rate | Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate			0.50%