Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Sep. 30, 2014
Segment Reporting Information
Revenues	$ 143,490	$ 152,694	$ 931,369	$ 965,549
Cost of sales	53,691	63,323	475,079	515,612
Depreciation and amortization	15,913	14,892	46,982	43,985
Operating income	20,184	19,720	238,523	243,517
Interest expense	9,985	10,433	31,245	28,036
Income before income taxes	10,199	9,287	207,278	215,481
Capital expenditures	43,315	38,215	139,624	104,117
Total assets (at period end)	2,423,205	2,289,841	2,423,205	2,289,841	$ 2,354,643
Goodwill (at period end)	182,145	182,145	182,145	182,145	$ 182,145
Reportable Segments | Gas Utility
Segment Reporting Information
Revenues	119,412	128,264	847,890	879,989
Cost of sales	41,352	49,257	426,715	463,492
Depreciation and amortization	14,786	13,774	43,555	40,733
Operating income	15,099	17,115	226,248	233,728
Interest expense	9,483	9,904	29,717	26,652
Income before income taxes	5,616	7,211	196,531	207,076
Capital expenditures	41,324	35,955	134,018	98,806
Total assets (at period end)	2,278,975	2,147,407	2,278,975	2,147,407
Goodwill (at period end)	182,145	182,145	182,145	182,145
Reportable Segments | Electric Utility
Segment Reporting Information
Revenues	23,875	23,954	82,621	84,467
Cost of sales	12,339	14,066	48,364	52,120
Depreciation and amortization	1,127	1,118	3,427	3,252
Operating income	4,071	2,304	11,300	9,485
Interest expense	502	529	1,528	1,384
Income before income taxes	3,569	1,775	9,772	8,101
Capital expenditures	1,991	2,260	5,606	5,311
Total assets (at period end)	144,230	142,434	144,230	142,434
Goodwill (at period end)	0	0	0	0
Other
Segment Reporting Information
Revenues	203	476	858	1,093
Cost of sales	0	0	0	0
Depreciation and amortization	0	0	0	0
Operating income	1,014	301	975	304
Interest expense	0	0	0	0
Income before income taxes	1,014	301	975	304
Capital expenditures	0	0	0	0
Total assets (at period end)	0	0	0	0
Goodwill (at period end)	$ 0	$ 0	$ 0	$ 0