EQUITY OF AMERICAN ASSETS TRUST, INC. (Details)				3 Months Ended		9 Months Ended
	Jun. 14, 2019 USD ($) shares	Mar. 02, 2018 Agent	May 27, 2015 USD ($) Agent	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) shares
Equity [Line Items]
Proceeds from issuance of common stock | $	$ 472,600,000					$ 513,934,000	$ (236,000)
Common stock, shares issued (in shares) | shares	10,925,000
Option Indexed to Issuer's Equity, Indexed Shares | shares	1,425,000
Noncash compensation expense | $				$ 1,100,000	$ 700,000	3,400,000	$ 2,200,000
Unrecognized compensation expense | $				$ 3,300,000		$ 3,300,000
At The Market Equity Program
Equity [Line Items]
Number of sales agents | Agent		1	5
Aggregate offering price of common share | $			$ 250,000,000.0
Common shares issued (shares) | shares				234,814		916,727
Weighted average price per share | $ / shares				$ 47.23		$ 45.99
Proceeds from issuance of common stock | $				$ 10,920,000		$ 41,309,000
Remaining capacity to issue | $				$ 134,000,000.0		$ 134,000,000.0
Operating Partnership Units
Equity [Line Items]
Antidilutive securities excluded from computation of EPS (in shares) | shares				329,652	262,605	331,245	265,762
American Assets Trust, L.P. | Operating Partnership Units
Equity [Line Items]
Weighted average unvested shares outstanding (in shares) | shares				329,652	262,605	331,245	265,762
Antidilutive securities excluded from computation of EPS (in shares) | shares				329,652	262,605	331,245	265,762