Equity of American Assets Trust, Inc. (Details)				12 Months Ended
	Jun. 14, 2019 USD ($) shares	May 27, 2015 USD ($) Agent	May 06, 2013 USD ($) Agent	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares
Equity [Line Items]
Preferred stock, shares authorized (in shares)				10,000,000
Preferred stock, par-value (in dollars per share) | $ / shares				$ 0.01
Preferred stock, shares issued (in shares)				0
Equity incentive award plan, common stock authorized for issuance (in shares)				4,054,411
Equity incentive award plan, common stock available for future issuance (in shares)				2,477,277
Noncash compensation expense | $				$ 6,300,000	$ 4,500,000	$ 3,000,000.0
Unrecognized compensation expense | $				8,400,000
Proceeds from issuance of common stock, net | $	$ 472,600,000			$ 0	$ 515,354,000	$ (236,000)
Common stock, shares issued	10,925,000			60,476,292	60,068,228
Option Indexed to Issuer's Equity, Indexed Shares	1,425,000
American Assets Trust, L.P.
Equity [Line Items]
Antidilutive securities excluded from computation of EPS (in shares)				357,048	330,197	271,905
Restricted Stock
Equity [Line Items]
Unrecognized compensation expense, weighted-average recognition period (in years)				1 year 7 months 6 days
At The Market Equity Program
Equity [Line Items]
Aggregate offering price of common share | $		$ 250,000,000.0	$ 150,000,000.0
Common shares issued (in shares)				0
Remaining capacity available for issuance | $				$ 132,600,000
$150.0 Million ATM | At The Market Equity Program
Equity [Line Items]
Number of sales agents | Agent			4
$250.0 Million ATM | At The Market Equity Program
Equity [Line Items]
Number of sales agents | Agent		5
Operating Partnership Units | American Assets Trust, L.P.
Equity [Line Items]
Weighted average unvested shares outstanding (in shares)				357,048	330,197	271,905