Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Segment	Jun. 30, 2012	Sep. 30, 2012
Segment Reporting [Abstract]
Number of reportable segments (in segments)			2
Segment Reporting Information [Line Items]
Revenues	$ 148,798	$ 143,644	$ 818,496	$ 770,087
Cost of sales	64,332	62,701	415,394	412,377
Depreciation and amortization	14,033	13,251	41,316	39,424
Operating income	18,878	25,300	200,672	178,352
Interest expense	9,690	10,575	29,587	31,859
Income before income taxes	9,188	14,725	171,085	146,493
Total assets (at period end)	2,328,056	2,185,845	2,328,056	2,185,845	2,223,179
Goodwill (at period end)	182,145	182,145	182,145	182,145	182,145
Capital expenditures	38,961	29,914	94,480	79,698
Gas Utility [Member]
Segment Reporting Information [Line Items]
Revenues	126,725	122,273	743,614	696,814
Cost of sales	52,365	51,401	372,681	370,572
Depreciation and amortization	13,012	12,332	38,348	36,635
Operating income	16,054	22,535	191,592	168,735
Interest expense	9,172	9,965	28,071	30,148
Income before income taxes	6,882	12,570	163,521	138,587
Total assets (at period end)	2,164,435	2,027,023	2,164,435	2,027,023
Goodwill (at period end)	182,145	182,145	182,145	182,145
Capital expenditures	37,224	29,004	90,173	76,470
Electric Utility [Member]
Segment Reporting Information [Line Items]
Revenues	21,535	20,779	73,604	71,888
Cost of sales	11,967	11,300	42,713	41,805
Depreciation and amortization	1,021	919	2,968	2,789
Operating income	2,584	2,554	8,597	9,147
Interest expense	518	610	1,516	1,711
Income before income taxes	2,066	1,944	7,081	7,436
Total assets (at period end)	163,621	158,822	163,621	158,822
Goodwill (at period end)	0	0	0	0
Capital expenditures	1,737	910	4,307	3,228
Other [Member]
Segment Reporting Information [Line Items]
Revenues	538	592	1,278	1,385
Cost of sales	0	0	0	0
Depreciation and amortization	0	0	0	0
Operating income	240	211	483	470
Interest expense	0	0	0	0
Income before income taxes	240	211	483	470
Total assets (at period end)	0	0	0	0
Goodwill (at period end)	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0