DEBT - Summary of Total Unsecured Notes Payable Outstanding (Details) - American Assets Trust, L.P. - USD ($) $ in Thousands							3 Months Ended
		Jun. 23, 2017	May 11, 2017	Jan. 18, 2017	Sep. 19, 2014	Aug. 19, 2014	Mar. 31, 2018	Dec. 31, 2017
Forward Contracts
Debt Instrument [Line Items]
Derivative contract term						7 years
Gain on derivative settlement			$ 700	$ 10,400	$ 1,600
Loss on derivative settlement		$ 500
Unsecured term loans, net
Debt Instrument [Line Items]
Long-term Debt, Principal Balance							$ 1,050,000	$ 1,050,000
Debt issuance costs, net							(4,822)	(4,530)
Total Debt Outstanding							1,045,178	1,045,470
Debt issuance costs, accumulated amortization							$ 6,159	5,866
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							Jan. 09, 2019
Long-term Debt, Principal Balance							$ 100,000	100,000
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	[1]						4.04%
Stated Maturity Date							Oct. 31, 2021
Long-term Debt, Principal Balance							$ 150,000	$ 150,000
Term Loan B | Interest Rate Swap
Debt Instrument [Line Items]
Derivative fixed interest rate (in percent)							2.75%	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	$ 100,000
Term Loan C | Interest Rate Swap
Debt Instrument [Line Items]
Derivative fixed interest rate (in percent)							2.74%	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	$ 50,000
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	[2]						3.78%
Stated Maturity Date							Jul. 19, 2024
Long-term Debt, Principal Balance							$ 100,000	100,000
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	100,000
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	100,000
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	[3]						4.29%
Stated Maturity Date							Mar. 01, 2027
Long-term Debt, Principal Balance							$ 250,000	250,000
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]						4.24%
Stated Maturity Date							May 23, 2029
Long-term Debt, Principal Balance							$ 100,000	$ 100,000

[1] 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.

[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 a highly effective cash flow hedge, 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.