Summary by Business Segment (Financial Information by Segment) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Net Sales		$ 1,374,569	$ 1,357,755	$ 2,724,279	$ 2,671,342
Cost of products sold (excluding depreciation)		601,173	600,534	1,198,339	1,183,940
Selling, distribution and administrative expenses		515,379	500,375	1,029,155	995,088
Depreciation		79,251	73,160	157,326	145,795
Amortization		8,621	7,905	16,736	15,657
Total costs and expenses		1,204,424	1,181,974	2,401,556	2,340,480
Operating Income		170,145	175,781	322,723	330,862
Gas and Rent [Member]
Segment Reporting Information [Line Items]
Net Sales		894,150	858,515	1,764,223	1,684,106
Hardgoods [Member]
Segment Reporting Information [Line Items]
Net Sales		480,419	499,240	960,056	987,236
Operating Segments [Member] | Distribution Business Segment [Member]
Segment Reporting Information [Line Items]
Net Sales		1,214,268	1,217,010	2,410,176	2,400,622
Cost of products sold (excluding depreciation)		530,578	531,867	1,051,818	1,052,800
Selling, distribution and administrative expenses		459,159	452,467	923,968	902,106
Depreciation		71,925	66,949	142,945	133,411
Amortization		7,430	6,881	14,592	13,605
Total costs and expenses		1,069,092	1,058,164	2,133,323	2,101,922
Operating Income		145,176	158,846	276,853	298,700
Operating Segments [Member] | Distribution Business Segment [Member] | Gas and Rent [Member]
Segment Reporting Information [Line Items]
Net Sales		734,852	718,743	1,452,494	1,415,308
Operating Segments [Member] | Distribution Business Segment [Member] | Hardgoods [Member]
Segment Reporting Information [Line Items]
Net Sales		479,416	498,267	957,682	985,314
Operating Segments [Member] | All Other Operations Business Segment [Member]
Segment Reporting Information [Line Items]
Net Sales		172,096	148,795	333,497	285,838
Cost of products sold (excluding depreciation)		82,390	76,717	165,915	146,258
Selling, distribution and administrative expenses		56,220	47,908	105,187	92,982
Depreciation		7,326	6,211	14,381	12,384
Amortization		1,191	1,024	2,144	2,052
Total costs and expenses		147,127	131,860	287,627	253,676
Operating Income		24,969	16,935	45,870	32,162
Operating Segments [Member] | All Other Operations Business Segment [Member] | Gas and Rent [Member]
Segment Reporting Information [Line Items]
Net Sales		171,087	147,821	331,114	283,913
Operating Segments [Member] | All Other Operations Business Segment [Member] | Hardgoods [Member]
Segment Reporting Information [Line Items]
Net Sales		1,009	974	2,383	1,925
Eliminations [Member]
Segment Reporting Information [Line Items]
Net Sales	[1]	(11,795)	(8,050)	(19,394)	(15,118)
Cost of products sold (excluding depreciation)	[1]	(11,795)	(8,050)	(19,394)	(15,118)
Selling, distribution and administrative expenses		0	0	0	0
Depreciation		0	0	0	0
Amortization		0	0	0	0
Total costs and expenses		(11,795)	(8,050)	(19,394)	(15,118)
Operating Income		0	0	0	0
Eliminations [Member] | Gas and Rent [Member]
Segment Reporting Information [Line Items]
Net Sales		(11,789)	(8,049)	(19,385)	(15,115)
Eliminations [Member] | Hardgoods [Member]
Segment Reporting Information [Line Items]
Net Sales		$ (6)	$ (1)	$ (9)	$ (3)

[1] Amounts in the “Eliminations” column represent the elimination of intercompany sales and associated gross profit on sales from the Company’s All Other Operations business segment to its Distribution business segment.