Equity of American Assets Trust, Inc. (Details)				12 Months Ended
	May 27, 2015 USD ($) Agent	Mar. 09, 2015 USD ($) $ / shares shares	May 06, 2013 USD ($) Agent	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) 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)				2,800,299
Noncash compensation expense | $				$ 4,700,000	$ 2,400,000	$ 2,900,000
Unrecognized compensation expense | $				$ 4,000,000
Weighted average unvested shares outstanding (in shares)				230,602	176,408	184,545
American Assets Trust, L.P.
Equity [Line Items]
Antidilutive securities excluded from computation of EPS (in shares)				230,602	176,408	184,545
Restricted Stock
Equity [Line Items]
Unrecognized compensation expense, weighted-average recognition period (in years)				1 year 7 months
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)				700,000
Weighted average price per share (in dollars per share) | $ / shares				$ 43.46
Gross proceeds from issuance of common stock | $				$ 30,400,000
Remaining capacity available for issuance | $				176,200,000
At The Market Equity Program | Commissions
Equity [Line Items]
Payments of stock issuance costs | $				(300,000)
At The Market Equity Program | Other Offering Costs
Equity [Line Items]
Payments of stock issuance costs | $				$ (250,000)
Private Placement | Explorer Insurance Company | Board of Directors Chairman
Equity [Line Items]
Common shares issued (in shares)		200,000
Gross proceeds from issuance of common stock | $		$ 8,100,000
Share price (in dollars per share) | $ / shares		$ 40.54
$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