Derivative and Hedging (Details) - USD ($)														12 Months Ended
	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	Dec. 31, 2018	Dec. 31, 2017	May 23, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative liabilities at fair value														$ (801,000)	$ (10,000)
Interest rate swap asset														$ 6,002,000	$ 5,091,000
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. | 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, 1/9/2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount														$ 100,000,000
Maturity date														Jan. 09, 2019
Effective date														Jan. 09, 2014
Interest rate swap asset														$ 18,000
Bank of America, N.A. | American Assets Trust, L.P. | Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Swap, 1/9/2021 [Member]
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														$ (801,000)
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
Interest rate swap asset														$ 3,976,000
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 | 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
Interest rate swap asset														$ 2,008,000
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														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														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, 2019