Equity of American Assets Trust, Inc. (Details) (USD $)	12 Months Ended			0 Months Ended		3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	May 06, 2013 Agent	Sep. 12, 2014	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Equity [Line Items]
Noncontrolling common units	17,905,257
Noncontrolling Interest, Ownership Percentage by Parent	70.70%
Percentage of ownership interests classified as noncontrolling	29.30%
Preferred stock, shares authorized	10,000,000
Preferred stock, par-value	$ 0.01
Equity incentive award plan, common stock authorized for issuance	4,054,411
Equity incentive award plan, common stock available for future issuance	3,560,872
Noncash compensation expense recognized	$ 3,700,000	$ 2,800,000	$ 2,900,000
Unrecognized compensation expense	5,100,000
Weighted average unvested shares outstanding	430,584	630,130	629,493
Restricted Stock
Equity [Line Items]
Unrecognized compensation expense, weighted-average recognition period (in years)	11 months 19 days
At The Market Equity Program
Equity [Line Items]
Number of Sales Agents				4
Aggregate Offering Price Of Common Share				150,000,000.0
Common shares issued (in shares)	2,710,067
Weighted average price per share	$ 33.84
Proceeds from Issuance of Common Stock, Gross	91,700,000
Remaining capacity available for issuance	32,300,000
Commissions | At The Market Equity Program
Equity [Line Items]
Payments of Stock Issuance Costs	(900,000)
Other Offering Costs | At The Market Equity Program
Equity [Line Items]
Payments of Stock Issuance Costs	(100,000)
ICW Plaza | Board of Directors Chairman | Private Placement
Equity [Line Items]
Common shares issued (in shares)					400,000
Share Price					$ 33.76
Proceeds from Issuance of Common Stock, Gross					$ 13,500,000
American Assets Trust, L.P.
Equity [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount						430,584	630,130	629,493