Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 485,738			$ 562,245			$ 2,458,165	$ 2,670,846
Cost of Revenue	31,326			130,886			845,167	1,124,763
Operation and maintenance expense	164,545			143,748			452,572	431,952
Depreciation and amortization expense	97,700			90,671			290,537	268,426
Taxes, other than income	69,965			72,620			213,546	208,400
Operating income	122,202			124,320			656,343	637,305
Other non-operating income (expense)	1,645			(3,330)			(1,846)	(8,054)
Interest charges	19,592			23,349			74,390	82,162
Income before income taxes	104,255			97,641			580,107	547,089
Income tax (benefit) expense	23,789			26,448			127,107	(17,228)
Net income	80,466	$ 214,888	$ 157,646	71,193	$ 178,992	$ 314,132	453,000	564,317
Capital expenditures	421,613			394,494			1,199,199	1,088,472
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	11,340,596						11,340,596		$ 10,371,147
Total assets	12,881,098						12,881,098		11,874,437
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	444,944			535,488			2,341,668	2,595,571
Operation and maintenance expense	123,998			110,568			347,386	343,860
Depreciation and amortization expense	70,611			66,504			210,224	197,587
Taxes, other than income	62,134			64,420			189,377	184,219
Operating income	48,683			63,109			447,083	448,207
Other non-operating income (expense)	3,005			(2,518)			1,791	(5,961)
Interest charges	10,597			13,315			44,703	51,581
Income before income taxes	41,091			47,276			404,171	390,665
Income tax (benefit) expense	8,693			11,932			85,195	(39,021)
Net income	32,398			35,344			318,976	429,686
Capital expenditures	316,825			284,209			912,640	749,693
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	8,404,238						8,404,238		7,644,693
Total assets	12,083,315						12,083,315		11,109,128
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	149,198			127,633			419,318	375,051
Operation and maintenance expense	40,855			33,494			106,120	89,027
Depreciation and amortization expense	27,089			24,167			80,313	70,839
Taxes, other than income	7,831			8,200			24,169	24,181
Operating income	73,519			61,211			209,260	189,098
Other non-operating income (expense)	(1,360)			(812)			(3,637)	(2,093)
Interest charges	8,995			10,034			29,687	30,581
Income before income taxes	63,164			50,365			175,936	156,424
Income tax (benefit) expense	15,096			14,516			41,912	21,793
Net income	48,068			35,849			134,024	134,631
Capital expenditures	104,788			110,285			286,559	338,779
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	2,936,358						2,936,358		2,726,454
Total assets	3,174,516						3,174,516		2,963,480
Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(108,404)			(100,876)			(302,821)	(299,776)
Operation and maintenance expense	(308)			(314)			(934)	(935)
Depreciation and amortization expense	0			0			0	0
Taxes, other than income	0			0			0	0
Operating income	0			0			0	0
Other non-operating income (expense)	0			0			0	0
Interest charges	0			0			0	0
Income before income taxes	0			0			0	0
Income tax (benefit) expense	0			0			0	0
Net income	0			0			0	0
Capital expenditures	0			0			0	0
Intersegment eliminations | Intersegment eliminations
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	0						0		0
Total assets	(2,376,733)						(2,376,733)		$ (2,198,171)
Reportable Subsegments
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	485,738			562,245			2,458,165	2,670,846
Reportable Subsegments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	444,287			534,816			2,339,660	2,593,578
Reportable Subsegments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	41,451			27,429			118,505	77,268
Reportable Subsegments | Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0			0			0	0
Intersubsegment Eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0			0			0	0
Intersubsegment Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	657			672			2,008	1,993
Intersubsegment Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	107,747			100,204			300,813	297,783
Intersubsegment Eliminations | Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(108,404)			(100,876)			(302,821)	(299,776)
Natural Gas, US Regulated
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	31,326			130,886			845,167	1,124,763
Natural Gas, US Regulated | Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	139,518			230,887			1,147,598	1,421,698
Natural Gas, US Regulated | Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	(96)			561			(544)	1,906
Natural Gas, US Regulated | Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	$ (108,096)			$ (100,562)			$ (301,887)	$ (298,841)