Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 877,782	$ 889,192
Operation and maintenance expense	138,600	129,045
Depreciation and amortization expense	96,065	88,374
Taxes, other than income	64,488	62,773
Operating income	236,464	242,083
Other non-operating expense	(7,723)	(2,557)
Interest charges	27,849	31,509
Income before income taxes	200,892	208,017
Income tax (benefit) expense	43,246	(106,115)
Net income	157,646	314,132
Capital expenditures	416,404	383,238
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	10,698,229		$ 10,371,147
Total assets	12,615,789		11,874,437
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	838,835	860,792
Operation and maintenance expense	105,767	103,215
Depreciation and amortization expense	69,709	65,434
Taxes, other than income	56,190	55,107
Operating income	169,437	173,278
Other non-operating expense	(6,477)	(1,922)
Interest charges	18,210	21,368
Income before income taxes	144,750	149,988
Income tax (benefit) expense	30,365	(99,111)
Net income	114,385	249,099
Capital expenditures	302,545	241,249
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	7,889,901		7,644,693
Total assets	11,836,888		11,109,128
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	134,470	126,463
Operation and maintenance expense	33,147	26,140
Depreciation and amortization expense	26,356	22,940
Taxes, other than income	8,298	7,666
Operating income	67,027	68,805
Other non-operating expense	(1,246)	(635)
Interest charges	9,639	10,141
Income before income taxes	56,142	58,029
Income tax (benefit) expense	12,881	(7,004)
Net income	43,261	65,033
Capital expenditures	113,859	141,989
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	2,808,328		2,726,454
Total assets	3,040,831		2,963,480
Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(95,523)	(98,063)
Operation and maintenance expense	(314)	(310)
Depreciation and amortization expense	0	0
Taxes, other than income	0	0
Operating income	0	0
Other non-operating expense	0	0
Interest charges	0	0
Income before income taxes	0	0
Income tax (benefit) expense	0	0
Net income	0	0
Capital expenditures	0	0
Intersegment eliminations | Intersegment eliminations
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	0		0
Total assets	(2,261,930)		$ (2,198,171)
Reportable Subsegments
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	877,782	889,192
Reportable Subsegments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	838,181	860,453
Reportable Subsegments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	39,601	28,739
Reportable Subsegments | Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0	0
Intersubsegment Eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0	0
Intersubsegment Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	654	339
Intersubsegment Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	94,869	97,724
Intersubsegment Eliminations | Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(95,523)	(98,063)
Natural Gas, US Regulated
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	342,165	366,917
Natural Gas, US Regulated | Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	437,732	463,758
Natural Gas, US Regulated | Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	(358)	912
Natural Gas, US Regulated | Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	$ (95,209)	$ (97,753)