Acquisitions (Summary of the Consideration Transferred and the Fair Values of the Assets Acquired and Liabilities Assumed) (Details) - USD ($) $ in Thousands		12 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Business Acquisition [Line Items]
Cash		$ 101,704	$ 51,382	$ 203,529
Recognized amounts of identifiable assets acquired and liabilities assumed
Goodwill		1,364,770	1,313,644	1,289,896
Series of Individually Immaterial Business Acquisitions [Member]
Business Acquisition [Line Items]
Cash	[1]	102,032	51,827	205,370
Airgas, Inc. common stock	[2]		4,458
Fair value of total consideration transferred			56,285
Recognized amounts of identifiable assets acquired and liabilities assumed
Current assets, net		10,760	9,708	14,640
Plant and equipment		24,947	13,824	48,173
Other intangible assets		36,826	16,162	60,190
Current liabilities		(11,463)	(5,815)	(4,722)
Non-current liabilities		(7,853)	(4,526)	(8,321)
Total identifiable net assets		53,217	29,353	109,960
Goodwill		48,815	26,932	95,410
Recognized amounts of identifiable assets acquired, goodwill, and liabilities assumed, net		102,032	$ 56,285	205,370
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		1,143,706	$ 1,116,569	1,092,728
Distribution Business Segment [Member] | Series of Individually Immaterial Business Acquisitions [Member]
Business Acquisition [Line Items]
Cash	[1]	58,018	50,774	204,957
Airgas, Inc. common stock	[2]		4,458
Fair value of total consideration transferred			55,232
Recognized amounts of identifiable assets acquired and liabilities assumed
Current assets, net		8,532	9,705	14,631
Plant and equipment		11,701	13,824	48,919
Other intangible assets		24,057	15,122	60,190
Current liabilities		(7,216)	(5,765)	(6,088)
Non-current liabilities		(3,856)	(4,526)	(8,321)
Total identifiable net assets		33,218	28,360	109,331
Goodwill		24,800	26,872	95,626
Recognized amounts of identifiable assets acquired, goodwill, and liabilities assumed, net		58,018	55,232	204,957
All Other Operations Business Segment [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed
Goodwill		221,064	197,075	197,168
All Other Operations Business Segment [Member] | Series of Individually Immaterial Business Acquisitions [Member]
Business Acquisition [Line Items]
Cash	[1]	44,014	1,053	413
Airgas, Inc. common stock	[2]		0
Fair value of total consideration transferred			1,053
Recognized amounts of identifiable assets acquired and liabilities assumed
Current assets, net		2,228	3	9
Plant and equipment		13,246	0	(746)
Other intangible assets		12,769	1,040	0
Current liabilities		(4,247)	(50)	1,366
Non-current liabilities		(3,997)	0	0
Total identifiable net assets		19,999	993	629
Goodwill		24,015	60	(216)
Recognized amounts of identifiable assets acquired, goodwill, and liabilities assumed, net		$ 44,014	$ 1,053	$ 413

[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.