Derivative and Hedging (Details) - USD ($)																			12 Months Ended
	Oct. 01, 2020	Aug. 11, 2020	Jul. 17, 2019	Jun. 20, 2019	Jan. 09, 2018	Jun. 23, 2017	May 31, 2017	May 11, 2017	Apr. 25, 2017	Mar. 31, 2017	Jan. 18, 2017	May 02, 2016	Apr. 07, 2016	Mar. 29, 2016	Mar. 23, 2016	Mar. 01, 2016	Jan. 29, 2016	Sep. 19, 2014	Dec. 31, 2020	Dec. 31, 2019	May 23, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months																			$ 1,300,000
Derivative liabilities at fair value																			$ (4,531,000)	$ (1,317,000)
Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount													$ 100,000,000	$ 150,000,000
Fixed interest rate on derivative													1.748%	1.88%
American Assets Trust, L.P. | Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Term of derivative contract							7 years		10 years				10 years	10 years					7 years
American Assets Trust, L.P. | Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Term of derivative contract																			1 month
Gain on derivatives	$ 0		$ 500,000					$ 700,000			$ 10,400,000							$ 1,600,000
American Assets Trust, L.P. | Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount															$ 50,000,000		$ 100,000,000		$ 100,000,000
Fixed interest rate on derivative															1.441%		1.4485%		4.13%
Term of derivative contract															7 years		7 years
Maturity date													Mar. 31, 2027	Mar. 31, 2027	Mar. 01, 2023		Mar. 01, 2023
Effective date													Mar. 31, 2017	Mar. 31, 2017	May 02, 2016		Mar. 01, 2016
Gain/Loss recognized in other comprehensive income (loss)											$ 10,400,000
American Assets Trust, L.P. | Cash Flow Hedging | Designated as Hedging Instrument | Treasury Lock
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount							$ 100,000,000		$ 100,000,000
Fixed interest rate on derivative							2.064%		2.313%
Maturity date							Jul. 26, 2017
Gain/Loss recognized in other comprehensive income (loss)						$ 500,000		$ 700,000
Gain on derivatives			$ 500,000
Bank of America, N.A. | American Assets Trust, L.P. | Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap, 1/9/2021
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount																			$ 100,000,000
Maturity date																			Jan. 09, 2021
Effective date																			Jan. 09, 2019
Derivative liabilities at fair value																			$ (85,000)
U.S. Bank N.A. | American Assets Trust, L.P. | Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap, 3/1/2023
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount																			$ 100,000,000
Maturity date																			Mar. 01, 2023
Effective date																			Mar. 01, 2016
Derivative liabilities at fair value																			$ (2,972,000)
Wells Fargo Bank, N.A. | American Assets Trust, L.P. | Not Designated as Hedging Instrument [Member] | Treasury Lock
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount		$ 200,000,000
Fixed interest rate on derivative		0.7115%
Term of derivative contract		10 years
Maturity date		Nov. 12, 2020
Wells Fargo Bank, N.A. | American Assets Trust, L.P. | Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap, 3/31/2027 $150M
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount														$ 150,000,000
Wells Fargo Bank, N.A. | American Assets Trust, L.P. | Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap, 3/31/2027 $100M
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount													$ 100,000,000
Wells Fargo Bank, N.A. | American Assets Trust, L.P. | Cash Flow Hedging | Designated as Hedging Instrument | Treasury Lock
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount				$ 100,000,000
Fixed interest rate on derivative				1.9925%
Term of derivative contract				10 years
Maturity date				Jul. 31, 2019
Amortization Period of Deferred Gain (Loss) on Discontinuation of Fair Value Hedge				10 years
Wells Fargo Bank, N.A. | American Assets Trust, L.P. | Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap, 3/1/2023
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount																			$ 50,000,000
Maturity date																			Mar. 01, 2023
Effective date																			May 02, 2016
Derivative liabilities at fair value																			$ (1,474,000)
Term Loan B | American Assets Trust, L.P.
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Stated Maturity Date																Mar. 01, 2023	Mar. 01, 2023
Face amount of debt																$ 100,000,000
Number of year amortization schedule					7 years											7 years
Term Loan B | American Assets Trust, L.P. | Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fixed interest rate on derivative																			2.75%	3.15%
Term Loan C | American Assets Trust, L.P.
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Stated Maturity Date												Mar. 01, 2023			Mar. 01, 2023
Face amount of debt												$ 50,000,000
Number of year amortization schedule					7 years							7 years
Term Loan C | American Assets Trust, L.P. | Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fixed interest rate on derivative																			2.74%	3.14%
Unsecured debt | American Assets Trust, L.P.
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of year amortization schedule										10 years
Term Loan A | American Assets Trust, L.P. | Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fixed interest rate on derivative																			4.13%
Unsecured debt | Term Loan B | American Assets Trust, L.P.
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Stated Maturity Date																			Mar. 01, 2023
Face amount of debt																$ 100,000,000
Unsecured debt | Term Loan C | American Assets Trust, L.P.
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Stated Maturity Date																			Mar. 01, 2023
Face amount of debt												$ 50,000,000
Unsecured debt | Senior Guaranteed Notes, Series E | American Assets Trust, L.P.
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Stated Maturity Date																			May 23, 2029
Face amount of debt																					$ 100,000,000
Number of year amortization schedule									12 years
Unsecured debt | Term Loan A | American Assets Trust, L.P.
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Stated Maturity Date																			Jan. 09, 2022