Document and Entity Information - shares	6 Months Ended
	Sep. 30, 2015	Nov. 02, 2015
Document Information [Line Items]
Document Type	10-Q
Document Period End Date	Sep. 30, 2015
Amendment Flag	false
Entity Registrant Name	AIRGAS INC
Trading Symbol	ARG
Entity Central Index Key	0000804212
Current Fiscal Year End Date	--03-31
Entity Filer Category	Large Accelerated Filer
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q2
Entity Common Stock, Shares Outstanding		72,057,506