Computation of Basic and Diluted EPS (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
NUMERATOR
Income from continuing operations	$ 4,865	$ 2,764
Less: Net income attributable to restricted shares	(132)	(132)
Less: Income from continuing operations attributable to unitholders in the Operating Partnership	(1,495)	(883)
Income from continuing operations attributable to American Assets Trust, Inc. common stockholders-basic	3,238	1,749
Plus: Results from discontinued operations attributable to American Assets Trust, Inc. common stockholders		107
NET INCOME ATTRIBUTABLE TO AMERICAN ASSETS TRUST, INC. STOCKHOLDERS	3,238	1,856
Income from continuing operations attributable to American Assets Trust, Inc. common stockholders-basic	3,238	1,749
Plus: Income from continuing operations attributable to unitholders in the Operating Partnership	1,495	883
Income from continuing operations attributable to common stockholders-diluted	4,733	2,632
Plus: Results from discontinued operations attributable to American Assets Trust, Inc. common stockholders		107
Plus: Results from discontinued operations attributable to unitholders in the Operating Partnership
Net income attributable to common stockholders-diluted	$ 4,733	$ 2,739
DENOMINATOR
Weighted average common shares outstanding-basic	39,033,013	38,657,170
Effect of dilutive securities-conversion of Operating Partnership units	18,023,435	18,396,089
Weighted average common shares outstanding - diluted	57,056,448	57,053,259
EARNINGS PER COMMON SHARE-BASIC
Continuing operations	$ 0.08	$ 0.05
Discontinued operations
Earnings per share-basic, Total	$ 0.08	$ 0.05
EARNINGS PER COMMON SHARE-DILUTED
Continuing operations	$ 0.08	$ 0.05
Discontinued operations
Earnings per share-diluted, Total	$ 0.08	$ 0.05