Debt Summary of Total Unsecured Notes Payable Outstanding (Details) - USD ($) $ in Thousands													12 Months Ended
		Jun. 23, 2017	May 31, 2017	May 11, 2017	Apr. 25, 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	Jul. 19, 2017	May 23, 2017	Mar. 01, 2017	Jan. 09, 2014
Debt Instrument [Line Items]
Unsecured notes payable													$ 1,045,863		$ 1,045,470
American Assets Trust, L.P.
Debt Instrument [Line Items]
Unsecured notes payable													$ 1,045,863		1,045,470
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				$ 700		$ 10,400							$ 1,600
Loss on derivatives		$ 500
Effective interest rate (percent)													3.88%
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%
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													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													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%
Unsecured debt | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term debt													$ 1,050,000		1,050,000
Debt issuance costs, net													(4,137)		(4,530)
Unsecured notes payable													1,045,863		1,045,470
Debt issuance costs, accumulated amortization													6,844		5,866
Unsecured debt | Term Loan A | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term debt													$ 100,000		100,000
Stated Maturity Date													Jan. 09, 2019
Unsecured debt | Senior Guaranteed Notes, Series A | American Assets Trust, L.P.
Debt Instrument [Line Items]
Long-term debt													$ 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													$ 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													$ 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													$ 100,000		100,000
Stated Interest Rate													3.78%			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													$ 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													$ 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													$ 250,000		250,000
Stated Interest Rate													4.29%					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													$ 100,000		$ 100,000
Stated Interest Rate													4.24%	[2]			4.24%
Stated Maturity Date													May 23, 2029

[1] The company 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 (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 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.