Quarterly Financial Information (Unaudited) Selected Quarterly Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information [Line Items]
Total revenue	$ 66,478us-gaap_Revenues	$ 67,343us-gaap_Revenues	$ 62,199us-gaap_Revenues	$ 63,980us-gaap_Revenues	$ 64,645us-gaap_Revenues	$ 65,318us-gaap_Revenues	$ 62,914us-gaap_Revenues	$ 62,180us-gaap_Revenues	$ 260,000us-gaap_Revenues	$ 255,057us-gaap_Revenues	$ 235,466us-gaap_Revenues
Operating income (loss)	22,526us-gaap_OperatingIncomeLoss	23,036us-gaap_OperatingIncomeLoss	17,726us-gaap_OperatingIncomeLoss	20,381us-gaap_OperatingIncomeLoss	20,407us-gaap_OperatingIncomeLoss	21,441us-gaap_OperatingIncomeLoss	19,373us-gaap_OperatingIncomeLoss	19,880us-gaap_OperatingIncomeLoss	83,669us-gaap_OperatingIncomeLoss	81,101us-gaap_OperatingIncomeLoss	71,906us-gaap_OperatingIncomeLoss
Income (loss) from continuing operations									31,145	22,594	13,949
Results from discontinued operations									0	0	37,652
Net income	10,046us-gaap_ProfitLoss	9,090us-gaap_ProfitLoss	5,351us-gaap_ProfitLoss	6,658us-gaap_ProfitLoss	6,907us-gaap_ProfitLoss	6,258us-gaap_ProfitLoss	4,564us-gaap_ProfitLoss	4,865us-gaap_ProfitLoss	31,145us-gaap_ProfitLoss	22,594us-gaap_ProfitLoss	51,601us-gaap_ProfitLoss
Net income attributable to restricted shares	(115)	(95)	(94)	(70)	(139)	(132)	(133)	(132)	(374)	(536)	(529)
Net income from continuing operations attributable to common stockholders-basic and diluted	$ 0.16	$ 0.15	$ 0.09	$ 0.11	$ 0.11	$ 0.11	$ 0.08	$ 0.08
Net income attributable to common stockholders-basic and diluted	$ 0.16us-gaap_EarningsPerShareBasicAndDiluted	$ 0.15us-gaap_EarningsPerShareBasicAndDiluted	$ 0.09us-gaap_EarningsPerShareBasicAndDiluted	$ 0.11us-gaap_EarningsPerShareBasicAndDiluted	$ 0.11us-gaap_EarningsPerShareBasicAndDiluted	$ 0.11us-gaap_EarningsPerShareBasicAndDiluted	$ 0.08us-gaap_EarningsPerShareBasicAndDiluted	$ 0.08us-gaap_EarningsPerShareBasicAndDiluted
American Assets Trust, L.P.
Quarterly Financial Information [Line Items]
Total revenue	66,478	67,343	62,199	63,980	64,645	65,318	62,914	62,180	260,000	255,057	235,466
Operating income (loss)	22,526	23,036	17,726	20,381	20,407	21,441	19,373	19,880	83,669	81,101	71,906
Income (loss) from continuing operations									31,145	22,594	13,949
Results from discontinued operations									0	0	37,652
Net income	10,046	9,090	5,351	6,658	6,907	6,258	4,564	4,865	31,145	22,594	51,601
Net income attributable to restricted shares	115	95	94	70	139	132	133	132	(374)	(536)	(529)
Net income from continuing operations attributable to common stockholders-basic and diluted	$ 0.16	$ 0.15	$ 0.09	$ 0.11	$ 0.11	$ 0.11	$ 0.08	$ 0.08
Net income attributable to common stockholders-basic and diluted	$ 0.16	$ 0.15	$ 0.09	$ 0.11	$ 0.11	$ 0.11	$ 0.08	$ 0.08
Net Income (Loss) Allocated to Limited Partners	(2,907)	(2,578)	(1,544)	(1,986)	(2,086)	(1,903)	(1,354)	(1,495)
Net Income (Loss) Allocated to General Partners	$ 7,024	$ 6,417	$ 3,713	$ 4,602	$ 4,682	$ 4,223	$ 3,077	$ 3,238

Net income attributable to restricted share.

No definition available.

Name:	aat_NetIncomeAttributableToRestrictedShare
Namespace Prefix:	aat_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	aat_QuarterlyFinancialInformationLineItems
Namespace Prefix:	aat_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

No definition available.

+ Details

Name:	us-gaap_EarningsPerShareBasicAndDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items, and noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 45

-Paragraph 19

-URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569616-111683

+ Details

Name:	us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of net income (loss) from continuing operations per each basic and diluted share of common stock or unit when the per share amount is the same for both basic and diluted shares.

+ References

No definition available.

+ Details

Name:	us-gaap_IncomeLossFromContinuingOperationsPerBasicAndDilutedShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of income (loss) from a disposal group, net of income tax before extraordinary items allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e957-107759

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.12)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 13

-Article 7

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.14)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 15

-Article 5

+ Details

Name:	us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Aggregate amount of net income allocated to general partners.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 4

-Section F

+ Details

Name:	us-gaap_NetIncomeLossAllocatedToGeneralPartners
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Aggregate amount of net income allocated to limited partners.

+ References

+ Details

Name:	us-gaap_NetIncomeLossAllocatedToLimitedPartners
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 55

-Paragraph 4K

-URI http://asc.fasb.org/extlink&oid=35736750&loc=SL4591552-111686

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 45

-Paragraph 19

-URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569616-111683

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (a),(c)

-URI http://asc.fasb.org/extlink&oid=18733093&loc=SL4573702-111684

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 55

-Paragraph 4J

-URI http://asc.fasb.org/extlink&oid=35736750&loc=SL4591551-111686

+ Details

Name:	us-gaap_ProfitLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ Details

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=aat_AmericanAssetsTrustL.P.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: