DEI Document (USD $) In Billions, except Share data, unless otherwise specified	12 Months Ended
	Mar. 31, 2014	May 27, 2014	Sep. 30, 2013
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Mar. 31, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	ARG
Entity Registrant Name	AIRGAS INC
Entity Central Index Key	0000804212
Current Fiscal Year End Date	--03-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		74,294,358
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Common Stock, Shares, Held by Non Affiliates			66,170,160
Entity Public Float			$ 7.0