DEI Document	3 Months Ended	9 Months Ended
	Dec. 31, 2012	Dec. 31, 2012	Feb. 06, 2013
Document Information [Line Items]
Document Type		10-Q
Amendment Flag		false
Document Period End Date		Dec. 31, 2012
Document Fiscal Year Focus		2013
Document Fiscal Period Focus		Q3
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			75,917,262
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes