Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2013 Segment	Dec. 31, 2012	Sep. 30, 2013
Segment Reporting [Abstract]
Number of reportable segments (in segments)	2
Segment Reporting Information [Line Items]
Revenues	$ 298,899	$ 273,797
Cost of sales	151,865	138,277
Depreciation and amortization	14,450	13,571
Operating income	85,843	72,564
Interest expense	8,797	10,077
Income before income taxes	77,046	62,487
Total assets (at period end)	2,326,670	2,298,995	2,210,322
Goodwill (at period end)	182,145	182,145	182,145
Capital expenditures	34,254	29,728
Gas Utility [Member]
Segment Reporting Information [Line Items]
Revenues	271,562	248,254
Cost of sales	135,487	123,574
Depreciation and amortization	13,384	12,605
Operating income	82,053	69,789
Interest expense	8,386	9,572
Income before income taxes	73,667	60,217
Total assets (at period end)	2,188,623	2,144,693
Goodwill (at period end)	182,145	182,145
Capital expenditures	32,812	28,519
Electric Utility [Member]
Segment Reporting Information [Line Items]
Revenues	26,961	25,118
Cost of sales	16,378	14,703
Depreciation and amortization	1,066	966
Operating income	3,689	2,624
Interest expense	411	505
Income before income taxes	3,278	2,119
Total assets (at period end)	138,047	154,302
Goodwill (at period end)	0	0
Capital expenditures	1,442	1,209
Other [Member]
Segment Reporting Information [Line Items]
Revenues	376	425
Cost of sales	0	0
Depreciation and amortization	0	0
Operating income	101	151
Interest expense	0	0
Income before income taxes	101	151
Total assets (at period end)	0	0
Goodwill (at period end)	0	0
Capital expenditures	$ 0	$ 0