Debt (Details)											12 Months Ended
	Jan. 26, 2021 USD ($)	Jan. 09, 2019 Extension_Option	Jan. 09, 2018 USD ($) Extension_Option	Apr. 25, 2017	May 02, 2016 USD ($)	Mar. 23, 2016	Mar. 01, 2016 USD ($)	Jan. 29, 2016	Oct. 31, 2014 USD ($)	Jan. 09, 2014 USD ($) Extension_Option	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 30, 2019	Jul. 19, 2017 USD ($)	May 23, 2017 USD ($)	Mar. 01, 2017 USD ($)
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate During Period											1.32%
Unsecured line of credit, net											$ 99,151,000	$ 0
Unsecured notes payable, net											1,196,677,000	1,195,780,000
Subsequent Event [Member]
Debt Instrument [Line Items]
Stated Interest Rate	3.375%
Unsecured line of credit
Debt Instrument [Line Items]
Maximum leverage ratio of revolving credit facility			60.00%
Maximum secured leverage ratio on revolving credit facility			40.00%
Minimum fixed charge coverage ratio covenant threshold			1.5							1.5
Minimum interest coverage ratio covenant threshold			175.00%							175.00%
Maximum unsecured leverage ratio (in percents)			60.00%
Minimum tangible net worth										$ 721,160,000
Percent of net proceeds of any additional equity issuances in relation to net proceeds as of Offering										75.00%
Maximum recourse indebtedness of total asset value (in percents)			15.00%
Percentage annual distributions cannot exceed funds from operations										95.00%
Unsecured line of credit | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			0.50%							0.50%
Unsecured line of credit | Eurodollar
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)										1.00%
Unsecured debt | Subsequent Event [Member]
Debt Instrument [Line Items]
Face amount of debt	$ 500,000,000
Term Loan A | Unsecured debt | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on federal funds rate		0.50%
Term Loan A | Unsecured debt | Eurodollar
Debt Instrument [Line Items]
Basis spread on Eurodollar rate		1.00%
Term Loan B | Unsecured debt | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on federal funds rate			0.50%				0.50%
Term Loan B | Unsecured debt | Eurodollar
Debt Instrument [Line Items]
Basis spread on Eurodollar rate			1.00%				1.00%
Term Loan C | Unsecured debt | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on federal funds rate			0.50%
Term Loan C | Unsecured debt | Eurodollar
Debt Instrument [Line Items]
Basis spread on Eurodollar rate			1.00%
Second Amended and Restated Credit Facility | Unsecured line of credit
Debt Instrument [Line Items]
Percentage annual distributions cannot exceed funds from operations			95.00%
Second Amended and Restated Credit Facility | Unsecured line of credit | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.00%
American Assets Trust, L.P.
Debt Instrument [Line Items]
Unsecured line of credit, net											99,151,000	0
Unsecured notes payable, net											1,196,677,000	1,195,780,000
American Assets Trust, L.P. | Unsecured line of credit | Base Rate
Debt Instrument [Line Items]
Variable rate basis										base rate
American Assets Trust, L.P. | Unsecured line of credit | Revolving Credit Facility
Debt Instrument [Line Items]
Number of extensions available | Extension_Option										2
Extension term (in months)										6 months
American Assets Trust, L.P. | Unsecured debt
Debt Instrument [Line Items]
Long-term Debt, Principal Balance											1,200,000,000	1,200,000,000
Unsecured notes payable, net											$ 1,196,677,000	1,195,780,000
American Assets Trust, L.P. | Term Loan A
Debt Instrument [Line Items]
Unsecured notes payable, net										$ 100,000,000
American Assets Trust, L.P. | Term Loan A | Minimum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)		0.20%
American Assets Trust, L.P. | Term Loan A | Maximum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)		0.70%
American Assets Trust, L.P. | Term Loan A | Unsecured debt
Debt Instrument [Line Items]
Stated Maturity Date											Jan. 09, 2022
Long-term Debt, Principal Balance											$ 100,000,000	100,000,000
Number of extensions available | Extension_Option		3
Extension term (in months)		1 year
American Assets Trust, L.P. | Term Loan A | Unsecured debt | LIBOR
Debt Instrument [Line Items]
Variable rate basis		LIBOR
American Assets Trust, L.P. | Term Loan A | Unsecured debt | Base Rate
Debt Instrument [Line Items]
Prime rate		prime rate
Federal funds rate		federal funds rate
Eurodollar		Eurodollar
American Assets Trust, L.P. | Term Loan A | Unsecured debt | Minimum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)		1.20%
American Assets Trust, L.P. | Term Loan A | Unsecured debt | Maximum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)		1.70%
American Assets Trust, L.P. | Term Loan B
Debt Instrument [Line Items]
Number of year amortization schedule			7 years				7 years
Face amount of debt							$ 100,000,000
Stated Maturity Date							Mar. 01, 2023	Mar. 01, 2023
American Assets Trust, L.P. | Term Loan B | Minimum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			0.70%				0.70%
American Assets Trust, L.P. | Term Loan B | Maximum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.35%				1.35%
American Assets Trust, L.P. | Term Loan B | Unsecured debt
Debt Instrument [Line Items]
Face amount of debt							$ 100,000,000
Stated Maturity Date											Mar. 01, 2023
Long-term Debt, Principal Balance											$ 100,000,000	100,000,000
American Assets Trust, L.P. | Term Loan B | Unsecured debt | LIBOR
Debt Instrument [Line Items]
Variable rate basis			LIBOR				LIBOR
American Assets Trust, L.P. | Term Loan B | Unsecured debt | Base Rate
Debt Instrument [Line Items]
Minimum base rate			0.00%				0.00%
Prime rate			prime rate				prime rate
Federal funds rate			federal funds rate				federal funds rate
Eurodollar			Eurodollar				Eurodollar
American Assets Trust, L.P. | Term Loan B | Unsecured debt | Minimum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.20%				1.70%
American Assets Trust, L.P. | Term Loan B | Unsecured debt | Maximum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.70%				2.35%
American Assets Trust, L.P. | Term Loan B & Term Loan C | Unsecured debt
Debt Instrument [Line Items]
Face amount of debt			$ 150,000,000
American Assets Trust, L.P. | Term Loan C
Debt Instrument [Line Items]
Number of year amortization schedule			7 years		7 years
Face amount of debt					$ 50,000,000
Stated Maturity Date					Mar. 01, 2023	Mar. 01, 2023
American Assets Trust, L.P. | Term Loan C | Minimum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			0.70%		0.70%
American Assets Trust, L.P. | Term Loan C | Maximum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.35%		1.35%
American Assets Trust, L.P. | Term Loan C | Unsecured debt
Debt Instrument [Line Items]
Face amount of debt					$ 50,000,000
Stated Maturity Date											Mar. 01, 2023
Long-term Debt, Principal Balance											$ 50,000,000	50,000,000
American Assets Trust, L.P. | Term Loan C | Unsecured debt | LIBOR
Debt Instrument [Line Items]
Variable rate basis			LIBOR		LIBOR
American Assets Trust, L.P. | Term Loan C | Unsecured debt | Base Rate
Debt Instrument [Line Items]
Minimum base rate			0.00%		0.00%
Prime rate			prime rate		prime rate
Federal funds rate			federal funds rate		federal funds rate
Eurodollar			Eurodollar		Eurodollar
American Assets Trust, L.P. | Term Loan C | Unsecured debt | Minimum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.20%		1.70%
American Assets Trust, L.P. | Term Loan C | Unsecured debt | Maximum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.70%		2.35%
American Assets Trust, L.P. | Note Purchase Agreement | Senior Guaranteed Notes
Debt Instrument [Line Items]
Face amount of debt									$ 350,000,000
Partial debt repayment, minimum percentage of principal (in percent)									5.00%
Full debt repayment percentage of principal plus a Make-Whole Amount (in percent)									100.00%
American Assets Trust, L.P. | Note Purchase Agreement | Senior Guaranteed Notes, Series A
Debt Instrument [Line Items]
Face amount of debt									$ 150,000,000
Stated Interest Rate									4.04%
American Assets Trust, L.P. | Note Purchase Agreement | Senior Guaranteed Notes, Series B
Debt Instrument [Line Items]
Face amount of debt									$ 100,000,000
Stated Interest Rate									4.45%
American Assets Trust, L.P. | Note Purchase Agreement | Senior Guaranteed Notes, Series C
Debt Instrument [Line Items]
Face amount of debt									$ 100,000,000
Stated Interest Rate									4.50%
American Assets Trust, L.P. | Amended and Restated Credit Facility
Debt Instrument [Line Items]
Revolving credit facility borrowing limit, maximum borrowing capacity										350,000,000
Allowable additional borrowing capacity										$ 250,000,000
American Assets Trust, L.P. | Amended and Restated Credit Facility | Minimum | Revolving Credit Facility | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)										0.35%
American Assets Trust, L.P. | Amended and Restated Credit Facility | Maximum | Revolving Credit Facility | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)										0.95%
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured line of credit | Base Rate
Debt Instrument [Line Items]
Prime rate										prime rate
Federal funds rate										federal funds rate
Eurodollar										Eurodollar
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured line of credit | Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility borrowing limit, maximum borrowing capacity										$ 250,000,000
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured line of credit | Revolving Credit Facility | LIBOR
Debt Instrument [Line Items]
Variable rate basis										LIBOR
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured line of credit | Minimum | Revolving Credit Facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)										1.35%
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured line of credit | Maximum | Revolving Credit Facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)										1.95%
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured debt
Debt Instrument [Line Items]
Revolving credit facility borrowing limit, maximum borrowing capacity										$ 100,000,000
Number of extensions available | Extension_Option										3
Extension term (in months)										12 months
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured debt | Minimum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)										1.30%
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured debt | Minimum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)										0.30%
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured debt | Maximum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)										1.90%
American Assets Trust, L.P. | Amended and Restated Credit Facility | Unsecured debt | Maximum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)										0.90%
American Assets Trust, L.P. | Senior Guaranteed Notes, Series D | Unsecured debt
Debt Instrument [Line Items]
Face amount of debt																$ 250,000,000
Stated Maturity Date											Mar. 01, 2027
Stated Interest Rate											4.29%					4.29%
Long-term Debt, Principal Balance											$ 250,000,000	250,000,000
American Assets Trust, L.P. | Senior Guaranteed Notes, Series E | Unsecured debt
Debt Instrument [Line Items]
Number of year amortization schedule				12 years
Face amount of debt															$ 100,000,000
Stated Maturity Date											May 23, 2029
Stated Interest Rate											4.24%				4.24%
Long-term Debt, Principal Balance											$ 100,000,000	100,000,000
American Assets Trust, L.P. | Senior Guaranteed Notes, Series F | Unsecured debt
Debt Instrument [Line Items]
Face amount of debt														$ 100,000,000
Stated Maturity Date											Jul. 19, 2024
Stated Interest Rate											3.78%			3.78%
Long-term Debt, Principal Balance											$ 100,000,000	100,000,000
American Assets Trust, L.P. | Senior Guaranteed Notes, Series G [Member] | Unsecured debt
Debt Instrument [Line Items]
Stated Maturity Date											Jul. 30, 2030
Stated Interest Rate											3.91%		3.91%
Long-term Debt, Principal Balance											$ 150,000,000	150,000,000
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
Allowable additional borrowing capacity			$ 250,000,000
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured line of credit
Debt Instrument [Line Items]
Unsecured line of credit, net											100,000,000
Debt issuance costs, net, revolving credit facility											$ 800,000
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured line of credit | Base Rate
Debt Instrument [Line Items]
Prime rate			prime rate
Federal funds rate			federal funds rate
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured line of credit | Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility borrowing limit, maximum borrowing capacity			$ 350,000,000
Number of extensions available | Extension_Option			2
Extension term (in months)			6 months
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured line of credit | Revolving Credit Facility | LIBOR
Debt Instrument [Line Items]
Variable rate basis			LIBOR
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured line of credit | Minimum | Revolving Credit Facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.05%
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured line of credit | Minimum | Revolving Credit Facility | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			0.10%
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured line of credit | Maximum | Revolving Credit Facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.50%
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured line of credit | Maximum | Revolving Credit Facility | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			0.50%
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured debt
Debt Instrument [Line Items]
Revolving credit facility borrowing limit, maximum borrowing capacity			$ 100,000,000
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured debt | Minimum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.30%
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured debt | Minimum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			0.30%
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured debt | Maximum | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			1.90%
American Assets Trust, L.P. | Second Amended and Restated Credit Facility | Unsecured debt | Maximum | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percents)			0.90%
American Assets Trust, L.P. | Senior Guaranteed Notes, Series A | Unsecured debt
Debt Instrument [Line Items]
Stated Maturity Date											Oct. 31, 2021
Stated Interest Rate											4.04%
Long-term Debt, Principal Balance											$ 150,000,000	$ 150,000,000
American Assets Trust, L.P. | Senior Guaranteed Notes, Series A | Unsecured debt | Subsequent Event [Member]
Debt Instrument [Line Items]
Repayments of Unsecured Debt	150,000,000.0
Gain (Loss) on Extinguishment of Debt	$ 3,900,000
Interest Rate Swap | American Assets Trust, L.P. | Term Loan A
Debt Instrument [Line Items]
Fixed interest rate on derivative											4.13%
Interest Rate Swap | American Assets Trust, L.P. | Term Loan B
Debt Instrument [Line Items]
Fixed interest rate on derivative											2.75%	3.15%
Interest Rate Swap | American Assets Trust, L.P. | Term Loan C
Debt Instrument [Line Items]
Fixed interest rate on derivative											2.74%	3.14%