Computation of Basic and Diluted EPS (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
NUMERATOR
Income from continuing operations	$ 4,822	$ 3,968	$ 2,396	$ 2,763	$ 560	$ (59)	$ (172)	$ 13,342	$ 13,949	$ 13,671	$ 1,622
Less: Net income attributable to restricted shares	(133)	(133)	(131)	(132)	(132)	(132)	(132)	(86)	(529)	(482)
Plus: Loss from continuing operations attributable to Predecessor's noncontrolling interests in consolidated real estate entities										2,455
Less: Income from continuing operations attributable to Predecessor's controlled owners' equity										(16,962)
Plus: Loss from continuing operations attributable to unitholders in the Operating Partnership									(4,239)	425
Income (loss) from continuing operations attributable to American Assets Trust, Inc. common stockholders-basic									9,181	(893)
Plus: Results from discontinued operations attributable to American Assets Trust, Inc. common stockholders									25,758	3,810
NET INCOME ATTRIBUTABLE TO AMERICAN ASSETS TRUST, INC. STOCKHOLDERS	28,577	2,817	1,689	1,856	344	3,052	219	(698)	34,939	2,917
Income (loss) from continuing operations attributable to American Assets Trust, Inc. common stockholders-basic									9,181	(893)
Less: Income (loss) from continuing operations attributable to unitholders in the Operating Partnership									4,239	(425)
Income (loss) from continuing operations attributable to common stockholders-diluted									13,420	(1,318)
Plus: Results from discontinued operations attributable to American Assets Trust, Inc. common stockholders									25,758	3,810
Plus: Results from discontinued operations attributable to unitholders in the Operating Partnership									11,894	1,813
Net income attributable to common stockholders-diluted									$ 51,072	$ 4,305
DENOMINATOR
Weighted average common shares outstanding-basic									38,736,113	36,748,806
Effect of dilutive securities-conversion of Operating Partnership units									18,317,796	17,471,001
Weighted average common shares outstanding - diluted									57,053,909	54,219,807
Earnings (loss) per common share-basic
Continuing operations									$ 0.24	$ (0.02)
Discontinued operations									$ 0.66	$ 0.10
Earnings per share-basic, Total									$ 0.90	$ 0.08
Earnings (loss) per common share-diluted
Continuing operations									$ 0.24	$ (0.02)
Discontinued operations									$ 0.66	$ 0.10
Earnings per share-diluted, Total									$ 0.90	$ 0.08