Segment Information (Details) $ in Thousands		3 Months Ended								12 Months Ended
		Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		$ 464,880	$ 526,501	$ 988,186	$ 780,168	$ 433,643	$ 456,212	$ 890,040	$ 674,753	$ 2,759,735	$ 2,454,648	$ 2,926,985
Purchased gas cost		72,561	114,176	427,494	311,305	63,927	64,583	377,356	240,326	925,536	746,192	1,295,675
Operating expenses
Operation and maintenance										546,798	538,592	516,406
Depreciation and amortization										319,448	290,791	272,408
Taxes, other than income										240,407	221,843	230,264
Operating income		91,792	140,664	285,172	209,918	84,449	128,396	251,656	192,729	727,546	657,230	612,232
Miscellaneous income (expense), net										(3,270)	(234)	(819)
Interest charges										120,182	114,812	116,241
Income from continuing operations before income taxes										604,094	542,184	495,172
Income tax expense										221,383	196,642	189,549
Income from Continuing Operations, Net of Tax		35,853	70,808	162,012	114,038	34,850	66,143	143,003	101,546	382,711	345,542	305,623
Income from discontinued operations, net of tax, before gain on disposal										10,994	4,562	9,452
Gain on sale from discontinued operations, net of tax										2,716	0	0
Net income from discontinued operations, net of tax										13,710	4,562	9,452
Net income		35,853	70,808	164,728	125,032	34,240	71,193	141,810	102,861	396,421	350,104	315,075
Capital expenditures										1,137,089	1,086,950	963,621
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	[1]	9,259,182				8,268,606				9,259,182	8,268,606
Total assets		10,749,596				10,010,889				10,749,596	10,010,889
Deferred income tax liabilities		1,878,699				1,603,056				$ 1,878,699	1,603,056
Number of Operating Segments | segment										3
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										$ 2,759,735	2,454,648	2,926,985
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										2,759,735	2,454,648	2,926,985
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										0	0	0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										0	0	0
Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		437,918	494,060	962,541	754,656	403,303	424,905	862,127	649,443	2,649,175	2,339,778	2,821,362
Purchased gas cost										1,269,456	1,058,576	1,574,447
Operating expenses
Operation and maintenance										413,077	407,982	393,504
Depreciation and amortization										249,071	234,109	224,094
Taxes, other than income										211,929	197,227	206,625
Operating income										505,642	441,884	422,692
Miscellaneous income (expense), net										(1,695)	1,171	284
Interest charges										79,789	78,238	83,087
Income from continuing operations before income taxes										424,158	364,817	339,889
Income tax expense										155,789	130,987	134,069
Income from Continuing Operations, Net of Tax										268,369	233,830	205,820
Income from discontinued operations, net of tax, before gain on disposal										0	0	0
Gain on sale from discontinued operations, net of tax										0
Net income										268,369	233,830	205,820
Capital expenditures										849,950	740,246	670,620
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment		6,849,517				6,208,465				6,849,517	6,208,465
Total assets		10,050,164				9,321,815				10,050,164	9,321,815
Deferred income tax liabilities		1,271,808				1,055,348				1,271,808	1,055,348
Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										2,647,813	2,338,404	2,819,977
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										2,647,813	2,338,404	2,819,977
Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										2,530,277	2,236,774	2,718,311
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										2,530,277	2,236,774	2,718,311
Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member] | Residential Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										1,642,918	1,477,049	1,761,689
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										1,642,918	1,477,049	1,761,689
Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member] | Commercial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										708,167	619,979	772,187
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										708,167	619,979	772,187
Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member] | Industrial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										133,372	98,439	131,034
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										133,372	98,439	131,034
Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Sales Revenues [Member] | Public Authority and Other Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										45,820	41,307	53,401
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										45,820	41,307	53,401
Distribution Segment [Member] | Reportable Subsegments [Member] | Gas Transportation Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										86,332	76,690	72,340
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										86,332	76,690	72,340
Distribution Segment [Member] | Reportable Subsegments [Member] | Other Gas Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										31,204	24,940	29,326
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										31,204	24,940	29,326
Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										1,362	1,374	1,385
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										1,362	1,374	1,385
Pipeline and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		117,823	117,283	111,972	109,952	112,772	113,855	102,153	98,416	457,030	427,196	384,957
Purchased gas cost										2,506	(58)	562
Operating expenses
Operation and maintenance										133,765	130,610	122,902
Depreciation and amortization										70,377	56,682	48,314
Taxes, other than income										28,478	24,616	23,639
Operating income										221,904	215,346	189,540
Miscellaneous income (expense), net										(1,575)	(1,405)	(1,103)
Interest charges										40,393	36,574	33,154
Income from continuing operations before income taxes										179,936	177,367	155,283
Income tax expense										65,594	65,655	55,480
Income from Continuing Operations, Net of Tax										114,342	111,712	99,803
Income from discontinued operations, net of tax, before gain on disposal										0	0	0
Gain on sale from discontinued operations, net of tax										0
Net income										114,342	111,712	99,803
Capital expenditures										287,139	346,383	292,775
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment		2,409,665				2,060,141				2,409,665	2,060,141
Total assets		2,621,601				2,283,864				2,621,601	2,283,864
Deferred income tax liabilities		606,891				543,390				606,891	543,390
Pipeline and Storage Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										111,922	116,244	107,008
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										111,922	116,244	107,008
Pipeline and Storage Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										345,108	310,952	277,949
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										345,108	310,952	277,949
Natural Gas Marketing Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues										0	0	0
Purchased gas cost										0	0	0
Operating expenses
Operation and maintenance										0	0	0
Depreciation and amortization										0	0	0
Taxes, other than income										0	0	0
Operating income										0	0	0
Miscellaneous income (expense), net										0	0	0
Interest charges										0	0	0
Income from continuing operations before income taxes										0	0	0
Income tax expense										0	0	0
Income from Continuing Operations, Net of Tax										0	0	0
Income from discontinued operations, net of tax, before gain on disposal										10,994	4,562	9,452
Gain on sale from discontinued operations, net of tax										2,716
Net income										13,710	4,562	9,452
Capital expenditures										0	321	226
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment		0				0				0	0
Total assets		0				216,715				0	216,715
Deferred income tax liabilities		0				4,318				0	4,318
Disposal Group, Including Discontinued Operation, Property, Plant and Equipment						11,900					11,900
Natural Gas Marketing Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										0	0	0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										0	0	0
Natural Gas Marketing Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										0	0	0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										0	0	0
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		(90,861)	$ (84,842)	$ (86,327)	$ (84,440)	(82,432)	$ (82,548)	$ (74,240)	$ (73,106)	(346,470)	(312,326)	(279,334)
Purchased gas cost										(346,426)	(312,326)	(279,334)
Operating expenses
Operation and maintenance										(44)	0	0
Depreciation and amortization										0	0	0
Taxes, other than income										0	0	0
Operating income										0	0	0
Miscellaneous income (expense), net										0	0	0
Interest charges										0	0	0
Income from continuing operations before income taxes										0	0	0
Income tax expense										0	0	0
Income from Continuing Operations, Net of Tax										0	0	0
Income from discontinued operations, net of tax, before gain on disposal										0	0	0
Gain on sale from discontinued operations, net of tax										0
Net income										0	0	0
Capital expenditures										0	0	0
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment		0				0				0	0
Total assets		(1,922,169)				(1,811,505)				(1,922,169)	(1,811,505)
Deferred income tax liabilities		$ 0				$ 0				0	0
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										0	0	0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										0	0	0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties										(346,470)	(312,326)	(279,334)
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties										$ (346,470)	$ (312,326)	$ (279,334)

[1] Net property, plant and equipment includes plant acquisition adjustments of $(64.1) million and $(59.8) million at September 30, 2017 and 2016.