Benefit Plans Benefit Plans (Deferred Compensation Plan) (Details) (USD $) In Millions, unless otherwise specified			12 Months Ended
Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Maximum [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Salary contribution to deferred compensation plan, percentage			75.00%
Bonus contribution to deferred compensation plan, percentage			100.00%
Deferred compensation plan assets	$ 16.4	$ 13.6
Deferred compensation plan liabilities	$ 16.4	$ 13.6

Deferred Compensation Plan Annual Percentage Bonus Compensation Contribution By Participants

No definition available.

Name:	arg_DeferredCompensationPlanAnnualPercentageBonusCompensationContributionByParticipants
Namespace Prefix:	arg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Deferred Compensation Plan Annual Percentage of Salary Contributions to Plan By Participants

+ References

No definition available.

+ Details

Name:	arg_DeferredCompensationPlanAnnualPercentageSalaryContributionByParticipants
Namespace Prefix:	arg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements payable beyond one year (or the operating cycle, if longer).

+ References

No definition available.

+ Details

Name:	us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.17)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 17

-Article 5

+ Details

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