Acquisitions and Divestitures (Summary of the Consideration Transferred and the Fair Values of the Assets Acquired and Liabilities Assumed) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Mar. 31, 2013
Consideration
Cash	$ 51,382		$ 203,529		$ 97,521
Recognized amounts of identifiable assets acquired and liabilities assumed
Goodwill	1,313,644		1,289,896		1,195,613
Series of Individually Immaterial Business Acquisitions [Member]
Consideration
Cash	51,827	[1]	205,370	[1]	97,521	[1]
Airgas, Inc. common stock	4,458	[2]
Fair value of total consideration transferred	56,285
Recognized amounts of identifiable assets acquired and liabilities assumed
Current assets, net	9,708		14,640		15,175
Plant and equipment	13,824		48,173		25,209
Other intangible assets	16,162		60,190		40,813
Current liabilities	(5,815)		(4,722)		(13,124)
Non-current liabilities	(4,526)		(8,321)		(4,757)
Total identifiable net assets	29,353		109,960		63,316
Goodwill	26,932		95,410		34,205
Recognized amounts of identifiable assets acquired, goodwill, and liabilities assumed, net	56,285		205,370		97,521
Number of shares of Airgas, Inc. common stock issued in connection with an acquisition	41,060
Distribution Business Segment [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed
Goodwill	1,116,569		1,092,728		998,128
Distribution Business Segment [Member] | Series of Individually Immaterial Business Acquisitions [Member]
Consideration
Cash	50,774	[1]	204,957	[1]	93,555	[1]
Airgas, Inc. common stock	4,458	[2]
Fair value of total consideration transferred	55,232
Recognized amounts of identifiable assets acquired and liabilities assumed
Current assets, net	9,705		14,631		14,627
Plant and equipment	13,824		48,919		24,191
Other intangible assets	15,122		60,190		38,658
Current liabilities	(5,765)		(6,088)		(10,990)
Non-current liabilities	(4,526)		(8,321)		(4,035)
Total identifiable net assets	28,360		109,331		62,451
Goodwill	26,872		95,626		31,104
Recognized amounts of identifiable assets acquired, goodwill, and liabilities assumed, net	55,232		204,957		93,555
All Other Operations Business Segment [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed
Goodwill	197,075		197,168		197,485
All Other Operations Business Segment [Member] | Series of Individually Immaterial Business Acquisitions [Member]
Consideration
Cash	1,053	[1]	413	[1]	3,966	[1]
Airgas, Inc. common stock	0	[2]
Fair value of total consideration transferred	1,053
Recognized amounts of identifiable assets acquired and liabilities assumed
Current assets, net	3		9		548
Plant and equipment	0		(746)		1,018
Other intangible assets	1,040		0		2,155
Current liabilities	(50)		1,366		(2,134)
Non-current liabilities	0		0		(722)
Total identifiable net assets	993		629		865
Goodwill	60		(216)		3,101
Recognized amounts of identifiable assets acquired, goodwill, and liabilities assumed, net	$ 1,053		$ 413		$ 3,966

[1] Includes cash paid, net of cash acquired, for current year acquisitions as well as payments for the settlement of holdback liabilities and contingent consideration arrangements associated with prior year acquisitions.

[2] Represents 41,060 shares of Airgas, Inc. common stock issued in connection with a single acquisition. The fair value of the shares issued as part of the consideration for the acquisition was determined based on the closing sales price of Airgas, Inc. common stock on the acquisition date.