SEGMENT REPORTING - Net Real Estate and Secured Note Payable Balances (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net Real Estate		$ 2,046,873	$ 2,039,853
Secured Notes Payable	[1]	162,856	182,765
American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Net Real Estate		2,046,873	2,039,853
Secured notes payable, net | American Assets Trust, L.P.
Segment Reporting Information [Line Items]
Debt issuance costs, net		168	193
Total Retail
Segment Reporting Information [Line Items]
Net Real Estate		633,605	628,734
Secured Notes Payable	[1]	34,812	35,008
Total Office
Segment Reporting Information [Line Items]
Net Real Estate		828,746	822,574
Secured Notes Payable	[1]	128,044	147,757
Total Multifamily
Segment Reporting Information [Line Items]
Net Real Estate		409,006	412,042
Secured Notes Payable	[1]	0	0
Total Mixed-Use
Segment Reporting Information [Line Items]
Net Real Estate		175,516	176,503
Secured Notes Payable	[1]	$ 0	$ 0

[1] Excludes debt issuance costs of $0.2 million and $0.2 million for each of the periods ending March 31, 2019 and December 31, 2018, respectively.