Equity of American Assets Trust, Inc. (Details)			12 Months Ended
	May 27, 2015 USD ($) Agent	May 06, 2013 USD ($) Agent	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) 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
Preferred stock, shares outstanding (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)			2,595,189
Noncash compensation expense | $			$ 3,000,000	$ 4,700,000	$ 2,400,000
Unrecognized compensation expense | $			$ 6,500,000
Weighted average unvested shares outstanding (in shares)			271,905	230,602	176,408
American Assets Trust, L.P.
Equity [Line Items]
Antidilutive securities excluded from computation of EPS (in shares)			271,905	230,602	176,408
Restricted Stock
Equity [Line Items]
Unrecognized compensation expense, weighted-average recognition period (in years)			1 year 7 months 10 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 | $			$ 176,200,000
$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