The amount retained from subcontractors and suppliers by the Company.

No definition available.

Name:	agx_AmountsRetainedFromVendorsAndSubcontractors
Namespace Prefix:	agx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Revenue recognition period.

+ References

No definition available.

+ Details

Name:	agx_RevenueRecognitionPeriod
Namespace Prefix:	agx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The amounts of unpriced change orders included in transaction prices that were used to determine revenues.

+ References

No definition available.

+ Details

Name:	agx_UnpricedChangeOrdersIncludedInTransactionPrices
Namespace Prefix:	agx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in measure of progress which (increases) decreases obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130551-203045

+ Details

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201409Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_AccountsPayableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

REVENUE RECOGNITION - Update to Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jul. 31, 2018	Jul. 31, 2018	Jan. 31, 2018
Update to Significant Accounting Policies
Amounts of unpriced change orders included in transaction prices	$ 4.1	$ 4.1	$ 9.3
Minimum
Update to Significant Accounting Policies
Revenue recognition period		3 months
Maximum
Update to Significant Accounting Policies
Revenue recognition period		3 years
Impact of the Adoption of ASC Topic 606
Update to Significant Accounting Policies
Impact on revenues, changes in estimates of the costs-to-complete active contracts	15.4	$ 10.8
Accounts payable
Update to Significant Accounting Policies
Amounts retained from vendors and subcontractors by the Company	$ 35.1	$ 35.1	$ 38.7