DEBT - Summary of Total Unsecured Notes Payable Outstanding (Details) - American Assets Trust, L.P.											9 Months Ended
		Jun. 23, 2017 USD ($)	May 31, 2017	May 11, 2017 USD ($)	Apr. 25, 2017	Jan. 18, 2017 USD ($)	Apr. 07, 2016	Mar. 29, 2016	Sep. 19, 2014 USD ($)	Aug. 19, 2014	Sep. 30, 2017 USD ($) Extension_Option		Jul. 19, 2017 USD ($)	May 23, 2017 USD ($)	Mar. 01, 2017 USD ($)	Dec. 31, 2016 USD ($)
Interest Rate Swap
Debt Instrument [Line Items]
Derivative contract term			7 years		10 years		10 years	10 years
Forward Contracts
Debt Instrument [Line Items]
Loss on derivative settlement		$ 500,000
Derivative contract term										7 years
Gain on derivative settlement				$ 700,000		$ 10,400,000			$ 1,600,000
Unsecured term loans, net
Debt Instrument [Line Items]
Long-term Debt, Principal Balance											$ 1,050,000,000					$ 600,000,000
Debt issuance costs, net											(5,066,000)					(3,650,000)
Total Debt Outstanding											1,044,934,000					596,350,000
Debt issuance costs, accumulated amortization											$ 5,473,000					4,317,000
Term Loan A
Debt Instrument [Line Items]
Debt extension options | Extension_Option											1
Term Loan A | Interest Rate Swap
Debt Instrument [Line Items]
Derivative fixed interest rate (in percent)											3.08%
Term Loan A | Unsecured term loans, net
Debt Instrument [Line Items]
Stated Maturity Date	[1]										Jan. 09, 2019
Long-term Debt, Principal Balance											$ 100,000,000					100,000,000
Senior Guaranteed Notes, Series A | Forward Contracts
Debt Instrument [Line Items]
Effective rate of debt instrument (in percent)											3.88%
Senior Guaranteed Notes, Series A | Unsecured term loans, net
Debt Instrument [Line Items]
Stated Interest Rate	[2]										4.04%
Stated Maturity Date											Oct. 31, 2021
Long-term Debt, Principal Balance											$ 150,000,000					150,000,000
Term Loan B | Interest Rate Swap
Debt Instrument [Line Items]
Derivative fixed interest rate (in percent)											3.15%
Term Loan B | Unsecured term loans, net
Debt Instrument [Line Items]
Stated Maturity Date											Mar. 01, 2023
Long-term Debt, Principal Balance											$ 100,000,000					100,000,000
Term Loan C | Interest Rate Swap
Debt Instrument [Line Items]
Derivative fixed interest rate (in percent)											3.14%
Term Loan C | Unsecured term loans, net
Debt Instrument [Line Items]
Stated Maturity Date											Mar. 01, 2023
Long-term Debt, Principal Balance											$ 50,000,000					50,000,000
Senior Guaranteed Notes, Series F | Forward Contracts
Debt Instrument [Line Items]
Effective rate of debt instrument (in percent)											3.85%
Senior Guaranteed Notes, Series F | Unsecured term loans, net
Debt Instrument [Line Items]
Stated Interest Rate											3.78%	[3]	3.78%
Face amount of debt													$ 100,000,000
Stated Maturity Date											Jul. 19, 2024
Long-term Debt, Principal Balance											$ 100,000,000					0
Senior Guaranteed Notes, Series B | Unsecured term loans, net
Debt Instrument [Line Items]
Stated Interest Rate											4.45%
Stated Maturity Date											Feb. 02, 2025
Long-term Debt, Principal Balance											$ 100,000,000					100,000,000
Senior Guaranteed Notes, Series C | Unsecured term loans, net
Debt Instrument [Line Items]
Stated Interest Rate											4.50%
Stated Maturity Date											Apr. 01, 2025
Long-term Debt, Principal Balance											$ 100,000,000					100,000,000
Senior Guaranteed Notes, Series D | Forward Contracts
Debt Instrument [Line Items]
Effective rate of debt instrument (in percent)											3.87%
Senior Guaranteed Notes, Series D | Unsecured term loans, net
Debt Instrument [Line Items]
Stated Interest Rate											4.29%	[4]			4.29%
Face amount of debt															$ 250,000,000
Stated Maturity Date											Mar. 01, 2027
Long-term Debt, Principal Balance											$ 250,000,000					0
Senior Guaranteed Notes, Series E | Forward Contracts
Debt Instrument [Line Items]
Effective rate of debt instrument (in percent)											4.18%
Senior Guaranteed Notes, Series E | Unsecured term loans, net
Debt Instrument [Line Items]
Stated Interest Rate											4.24%	[5]		4.24%
Face amount of debt														$ 100,000,000
Stated Maturity Date											May 23, 2029
Long-term Debt, Principal Balance											$ 100,000,000					$ 0

[1] The Operating Partnership has an option to extend the term loan up to one time, with such extension for a 12-month period. The foregoing extension option is exercisable by us subject to the satisfaction of certain conditions.

[2] The Operating Partnership entered into a one-month forward-starting seven-year swap contract on August 19, 2014, which was settled on September 19, 2014 at a gain of approximately $1.6 million. 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.

[3] 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.

[4] 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 a highly effective cash flow hedge, accordingly, the effective interest rate is approximately 3.87% per annum.

[5] 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.