Segment Information (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2020 USD ($) state	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2020 USD ($) state	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of states with service areas | state	8						8
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 492,995			$ 485,738			$ 2,346,223	$ 2,458,165
Purchased gas cost	26,072			31,326			640,823	845,167
Operation and maintenance expense	149,460			164,545			449,529	452,572
Depreciation and amortization expense	107,104			97,700			318,082	290,537
Taxes, other than income	71,324			69,965			214,535	213,546
Operating income	139,035			122,202			723,254	656,343
Other non-operating income	7,235			1,645			9,133	(1,846)
Interest charges	19,580			19,592			68,980	74,390
Income before income taxes	126,690			104,255			663,407	580,107
Income tax (benefit) expense	8,899			23,789			127,297	127,107
Net income	117,791	$ 239,646	$ 178,673	80,466	$ 214,888	$ 157,646	536,110	453,000
Capital expenditures	410,936			421,613			1,405,673	1,199,199
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	12,880,850						12,880,850		$ 11,787,669
Total assets	14,870,982						14,870,982		13,367,619
Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	434,650			444,287			2,194,786	2,339,660
Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	58,345			41,451			151,437	118,505
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	435,308			444,944			2,196,817	2,341,668
Purchased gas cost	126,093			139,518			942,586	1,147,598
Operation and maintenance expense	107,537			123,998			337,740	347,386
Depreciation and amortization expense	77,187			70,611			229,526	210,224
Taxes, other than income	61,980			62,134			190,636	189,377
Operating income	62,511			48,683			496,329	447,083
Other non-operating income	5,167			3,005			1,930	1,791
Interest charges	7,969			10,597			35,128	44,703
Income before income taxes	59,709			41,091			463,131	404,171
Income tax (benefit) expense	810			8,693			87,411	85,195
Net income	58,899			32,398			375,720	318,976
Capital expenditures	342,385			316,825			1,119,945	912,640
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	9,644,503						9,644,503		8,737,590
Total assets	14,098,715						14,098,715		12,579,741
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	158,008			149,198			452,421	419,318
Purchased gas cost	(11)			(96)			290	(544)
Operation and maintenance expense	42,234			40,855			112,751	106,120
Depreciation and amortization expense	29,917			27,089			88,556	80,313
Taxes, other than income	9,344			7,831			23,899	24,169
Operating income	76,524			73,519			226,925	209,260
Other non-operating income	2,068			(1,360)			7,203	(3,637)
Interest charges	11,611			8,995			33,852	29,687
Income before income taxes	66,981			63,164			200,276	175,936
Income tax (benefit) expense	8,089			15,096			39,886	41,912
Net income	58,892			48,068			160,390	134,024
Capital expenditures	68,551			104,788			285,728	286,559
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	3,236,347						3,236,347		3,050,079
Total assets	3,471,282						3,471,282		3,279,323
Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(100,321)			(108,404)			(303,015)	(302,821)
Purchased gas cost	(100,010)			(108,096)			(302,053)	(301,887)
Operation and maintenance expense	(311)			(308)			(962)	(934)
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	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
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	0						0		0
Total assets	(2,699,015)						(2,699,015)		$ (2,491,445)
Intersegment eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	658			657			2,031	2,008
Intersegment eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 99,663			$ 107,747			$ 300,984	$ 300,813