Debt Summary of Total Unsecured Notes Payable Outstanding (Details) $ in Thousands																12 Months Ended
		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 ($)	Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)	Jul. 19, 2017	May 23, 2017	Mar. 01, 2017	Jan. 09, 2014 USD ($)
Debt Instrument [Line Items]
Debt issuance costs, net																$ (1,256)		$ 0
Unsecured notes payable																1,195,780		1,045,863
Debt issuance costs, accumulated amortization																814		0
American Assets Trust, L.P.
Debt Instrument [Line Items]
Unsecured notes payable																$ 1,195,780		$ 1,045,863
American Assets Trust, L.P. | Interest Rate Swap
Debt Instrument [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
Debt Instrument [Line Items]
Term of derivative contract																1 month
Gain on derivatives		$ 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 | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term Debt, Principal Balance																$ 1,200,000		$ 1,050,000
Debt issuance costs, net																(4,220)		(4,137)
Unsecured notes payable																1,195,780		1,045,863
Debt issuance costs, accumulated amortization																7,835		6,844
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, 2021
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	[1]															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%	[2]		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%	[3]				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]			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		$ 0
Stated Interest Rate	[5]															3.91%
Stated Maturity Date																Jul. 30, 2030

[1] The company entered into a one month forward-starting seven years swap contract on August 19, 2014, which was settled on September 19, 2014 at a gain of approximately $1.6 million (see Note 8). The forward-starting seven-year swap contract was deemed to be a highly effective cash flow hedge, accordingly, the effective interest rate is approximately 3.88% per annum.

[2] The Operating Partnership entered into a treasury lock contract on May 31, 2017, which was settled on June 23, 2017 at a loss of approximately $0.5 million. The treasury lock contract was deemed to be a highly effective cash flow hedge, accordingly, the effective interest rate is approximately 3.85% per annum.

[3] The Operating Partnership entered into forward-starting interest rate swap contracts on March 29, 2016 and April 7, 2016, which were settled on January 18, 2017 at a gain of approximately $10.4 million. The forward-starting interest swap rate contracts were deemed to be highly effective cash flow hedges, accordingly, the effective interest rate is approximately 3.87% per annum.

[4] The Operating Partnership entered into a treasury lock contract on April 25, 2017, which was settled on May 11, 2017 at a gain of approximately $0.7 million. The treasury lock contract was deemed to be a highly effective cash flow hedge, accordingly, the effective interest rate is approximately 4.18% per annum.

[5] The Operating Partnership entered into a treasury lock contract on June 20, 2019, which was settled on July 17, 2019 at a gain of approximately $0.5 million. The treasury lock contract was deemed to be a highly effective cash flow hedge, accordingly, the effective interest rate is approximately 3.88% per annum.