Derivative and Hedging (Details) - USD ($)																	12 Months Ended
	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, 2019	Dec. 31, 2018	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																	(1,317,000)	$ (801,000)
Derivative assets at fair value																	$ 434,000	$ 6,002,000
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]
Fixed interest rate on derivative																	3.08%
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	$ 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%		3.08%
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
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
Loss recognized in other comprehensive income (loss)				$ 500,000		$ 700,000
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																	$ (1,317,000)
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 assets at fair value																	$ 279,000
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 assets at fair value																	$ 155,000
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, 2021