SEGMENT REPORTING - Net Real Estate and Secured Note Payable Balances (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net Real Estate		$ 2,514,297	$ 2,039,853
Secured Notes Payable	[1]	162,298	182,765
Debt issuance costs, net of accumulated amortization of $713 and $0, respectively		1,357	0
American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Net Real Estate		2,514,297	2,039,853
Secured notes payable, net | American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Debt issuance costs, net of accumulated amortization of $713 and $0, respectively		139	193
Retail
Segment Reporting Information [Line Items]
Net Real Estate		626,737	628,734
Secured Notes Payable	[1]	34,434	35,008
Office
Segment Reporting Information [Line Items]
Net Real Estate		1,306,967	822,574
Secured Notes Payable	[1]	127,864	147,757
Multifamily
Segment Reporting Information [Line Items]
Net Real Estate		403,555	412,042
Mixed-use
Segment Reporting Information [Line Items]
Net Real Estate		$ 177,038	$ 176,503

[1] Excludes debt issuance costs of $0.1 million and $0.2 million for each of the periods ended September 30, 2019 and December 31, 2018, respectively.