v2.4.0.8

ESOP Expense - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended					12 Months Ended
ESOP Expense - Additional Information (Detail) (USD $)	Jun. 30, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2013 ESOP [Member]	Mar. 31, 2013 ESOP [Member]	Sep. 30, 2011 Common Shares Issued for Stock Options [Member]
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
ESOP plan scheme		The Company matches the first 3% and one-half of the next 2% of eligible employee salary deferrals by contributing shares of Alion common stock to the ESOP Trust on March 31 and September 30 each year.
Percentage of employer contribution as a percentage of employee contribution deferral		3.00%
Percentage of salary		2.00%
Eligible employee compensation in common stock	1.00%	2.50%
Eligible employee compensation in cash	1.50%
Recognized ESOP expense		$ 13,800,000	$ 13,800,000	$ 13,200,000
Prior year expense				1,200,000			11,000,000
Employee funds used for purchase of common stock		930,000
Estimated fair value of share price					$ 8.10	$ 16.25
Redeemable common stock, issued value		$ 13,800,000

Employee stock ownership plan ESOP compensation expense as cash for prior period.

+ References

No definition available.

+ Details

Name:	alst_EmployeeStockOwnershipPlanEsopCompensationExpenseAsCashForPriorPeriod
Namespace Prefix:	alst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Employee stock ownership plan ESOP employee salary deferral employer matching percentage description.

+ References

No definition available.

+ Details

Name:	alst_EmployeeStockOwnershipPlanEsopEmployeeSalaryDeferralEmployerMatchingPercentageDescription
Namespace Prefix:	alst_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Employee stock ownership plan ESOP profit contribution as cash to trust percentage.

+ References

No definition available.

+ Details

Name:	alst_EmployeeStockOwnershipPlanEsopProfitContributionAsCashToTrustPercentage
Namespace Prefix:	alst_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Employee stock ownership plan ESOP profit contribution as stock to trust percentage.

+ References

No definition available.

+ Details

Name:	alst_EmployeeStockOwnershipPlanEsopProfitContributionAsStockToTrustPercentage
Namespace Prefix:	alst_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of employer contribution as percentage of employee contribution deferral.

+ References

No definition available.

+ Details

Name:	alst_PercentageOfEmployerContributionAsPercentageOfEmployeeContributionDeferral
Namespace Prefix:	alst_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Value of common stock issued to a trust (for example, a 'rabbi trust') set up specifically to accumulate stock for the sole purpose of distribution to participating employees. This trust does not allow employees to immediately or after a holding period diversify into nonemployer securities. The deferred compensation plan for which this trust is set up must be settled by the delivery of a fixed number of shares of employer stock.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 710

-SubTopic 10

-Section 25

-Paragraph 16

-URI http://asc.fasb.org/extlink&oid=6409836&loc=d3e19833-108362

+ Details

Name:	us-gaap_CommonStockIssuedEmployeeStockTrust
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of plan compensation cost recognized during the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 40

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929

+ Details

Name:	us-gaap_EmployeeStockOwnershipPlanESOPCompensationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Value of new stock classified as temporary equity issued during the period.

+ References

No definition available.

+ Details

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

Alion - IPS Corp S-1