Debt Summary of Total Unsecured Notes Payable Outstanding (Details) $ in Thousands																12 Months Ended
	Oct. 01, 2020 USD ($)	Jul. 17, 2019 USD ($)	Jan. 09, 2019 Extension_Option	Jun. 23, 2017 USD ($)	May 31, 2017	May 11, 2017 USD ($)	Apr. 25, 2017	Jan. 18, 2017 USD ($)	May 02, 2016	Apr. 07, 2016	Mar. 29, 2016	Mar. 23, 2016	Mar. 01, 2016	Jan. 29, 2016	Sep. 19, 2014 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 30, 2019	Jul. 19, 2017	May 23, 2017	Mar. 01, 2017	Jan. 09, 2014 USD ($)
Debt Instrument [Line Items]
Debt issuance costs, net																$ 0	$ (1,256)
Unsecured notes payable																1,196,677	1,195,780
Debt issuance costs, accumulated amortization																0	814
American Assets Trust, L.P.
Debt Instrument [Line Items]
Unsecured notes payable																$ 1,196,677	$ 1,195,780
American Assets Trust, L.P. | Interest Rate Swap
Debt Instrument [Line Items]
Term of derivative contract					7 years		10 years			10 years	10 years					7 years
American Assets Trust, L.P. | Forward Contracts
Debt Instrument [Line Items]
Term of derivative contract																1 month
Gain on derivatives	$ 0	$ 500				$ 700		$ 10,400							$ 1,600
Loss on derivatives				$ 500
Term Loan A | American Assets Trust, L.P.
Debt Instrument [Line Items]
Unsecured notes payable																						$ 100,000
Term Loan A | American Assets Trust, L.P. | Interest Rate Swap
Debt Instrument [Line Items]
Fixed interest rate on derivative																4.13%
Senior Guaranteed Notes, Series A | American Assets Trust, L.P. | Forward Contracts
Debt Instrument [Line Items]
Effective interest rate (percent)																3.88%
Term Loan B | American Assets Trust, L.P.
Debt Instrument [Line Items]
Stated Maturity Date													Mar. 01, 2023	Mar. 01, 2023
Term Loan B | American Assets Trust, L.P. | Interest Rate Swap
Debt Instrument [Line Items]
Fixed interest rate on derivative																2.75%	3.15%
Term Loan C | American Assets Trust, L.P.
Debt Instrument [Line Items]
Stated Maturity Date									Mar. 01, 2023			Mar. 01, 2023
Term Loan C | American Assets Trust, L.P. | Interest Rate Swap
Debt Instrument [Line Items]
Fixed interest rate on derivative																2.74%	3.14%
Senior Guaranteed Notes, Series F | American Assets Trust, L.P. | Forward Contracts
Debt Instrument [Line Items]
Effective interest rate (percent)																3.85%
Senior Guaranteed Notes, Series D | American Assets Trust, L.P. | Forward Contracts
Debt Instrument [Line Items]
Effective interest rate (percent)																3.87%
Senior Guaranteed Notes, Series E | American Assets Trust, L.P. | Forward Contracts
Debt Instrument [Line Items]
Effective interest rate (percent)																4.18%
Senior Guaranteed Notes, Series G [Member] | American Assets Trust, L.P. | Forward Contracts
Debt Instrument [Line Items]
Effective interest rate (percent)																3.88%
Unsecured debt
Debt Instrument [Line Items]
Debt issuance costs, accumulated amortization																$ 345	$ 449
Unsecured debt | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																1,200,000	1,200,000
Debt issuance costs, net																(3,323)	(4,220)
Unsecured notes payable																1,196,677	1,195,780
Debt issuance costs, accumulated amortization																8,856	7,835
Unsecured debt | Term Loan A | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 100,000	100,000
Stated Maturity Date																Jan. 09, 2022
Number of extensions available | Extension_Option			3
Extension term (in months)			1 year
Unsecured debt | Senior Guaranteed Notes, Series A | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 150,000	150,000
Stated Interest Rate																4.04%
Stated Maturity Date																Oct. 31, 2021
Unsecured debt | Term Loan B | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 100,000	100,000
Stated Maturity Date																Mar. 01, 2023
Unsecured debt | Term Loan C | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 50,000	50,000
Stated Maturity Date																Mar. 01, 2023
Unsecured debt | Senior Guaranteed Notes, Series F | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 100,000	100,000
Stated Interest Rate																3.78%			3.78%
Stated Maturity Date																Jul. 19, 2024
Unsecured debt | Senior Guaranteed Notes, Series B | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 100,000	100,000
Stated Interest Rate																4.45%
Stated Maturity Date																Feb. 02, 2025
Unsecured debt | Senior Guaranteed Notes, Series C | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 100,000	100,000
Stated Interest Rate																4.50%
Stated Maturity Date																Apr. 01, 2025
Unsecured debt | Senior Guaranteed Notes, Series D | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 250,000	250,000
Stated Interest Rate																4.29%					4.29%
Stated Maturity Date																Mar. 01, 2027
Unsecured debt | Senior Guaranteed Notes, Series E | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 100,000	100,000
Stated Interest Rate																4.24%				4.24%
Stated Maturity Date																May 23, 2029
Unsecured debt | Senior Guaranteed Notes, Series G [Member] | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 150,000	$ 150,000
Stated Interest Rate																3.91%		3.91%
Stated Maturity Date																Jul. 30, 2030