Acquisitions (Narrative) (Details) $ in Thousands			1 Months Ended									3 Months Ended							4 Months Ended	9 Months Ended		12 Months Ended
	Aug. 06, 2021 USD ($) customer	Mar. 16, 2020 USD ($) customer	Oct. 31, 2021 USD ($) customer	Jul. 31, 2021 USD ($) customer	Apr. 30, 2021 USD ($) customer	Jan. 31, 2021 USD ($) customer	Dec. 31, 2020 USD ($) customer	Oct. 31, 2020 USD ($) customer	Sep. 30, 2020 USD ($) customer	Jan. 31, 2020 USD ($) customer	Sep. 30, 2019 USD ($) customer item	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Business Acquisition [Line Items]
Goodwill							$ 2,324,547					$ 2,340,836			$ 2,324,547					$ 2,340,836		$ 2,324,547
Operating revenues												361,860				$ 348,647				1,342,457	$ 988,700
Net income												50,503	$ 80,914	$ 183,689		55,732	$ 74,629	$ 51,781		$ 315,106	$ 182,142
Peoples Gas Acquisition [Member]
Business Acquisition [Line Items]
Number of customers | customer		750,000
Total purchase price		$ 3,465,344
Indebtedness		1,101,091
short-term debt		181,000
Outstanding debt		920,091
Acquisition-related costs							20,628								20,628							20,628
Acquisition-related costs, pre-tax							$ 25,197								25,197							25,197
Adjustments to acquisition financing																						10,700
Adjustments to acquisition financing, Pre-tax																						14,342
Adjustments to debt																						2,448
Adjustments to debt, pre-tax																						3,442
Elimination of management fee																						885
Elimination of management fee, pre-tax																						1,245
Pipeline term																				7 years
Pipeline cost												120,000								$ 120,000
Customer rate credits												$ 23,004								$ 23,004
Property, plant and equipment, net		2,476,551
Goodwill		2,277,447											$ 16,400
Transaction-related expenses																		$ 25,397
Operating revenues																			$ 280,300
Net income																			$ 6,131
Illinois [Member] | Water Utility System [Member]
Business Acquisition [Line Items]
Number of customers | customer	6,500
Total purchase price	$ 32,100
Illinois [Member] | Water And Wastewater Utility Systems [Member]
Business Acquisition [Line Items]
Number of customers | customer					2,121			514
Total purchase price					$ 12,500			$ 4,859
Pennsylvania [Member] | Water Utility System [Member]
Business Acquisition [Line Items]
Number of customers | customer				2,930
Total purchase price				$ 12,000
Pennsylvania [Member] | Water And Wastewater Utility Systems [Member]
Business Acquisition [Line Items]
Number of customers | customer									4,947
Total purchase price									$ 21,000
Pennsylvania [Member] | Wastewater Utility System [Member]
Business Acquisition [Line Items]
Number of customers | customer						10,500	1,965		11,000
Total purchase price						$ 72,400	$ 29,944		$ 53,000
Delaware [Member] | Wastewater Utility System [Member]
Business Acquisition [Line Items]
Number of customers | customer											16,000
Retail customers | customer											198,000
Municipalities | item											42
Total purchase price											$ 276,500
Ohio [Member] | Water And Wastewater Utility Systems [Member]
Business Acquisition [Line Items]
Number of customers | customer										3,126
Total purchase price										$ 7,472
Texas [Member] | Water Utility System [Member]
Business Acquisition [Line Items]
Number of customers | customer	992
Total purchase price	$ 4,000
Subsequent Event [Member] | Illinois [Member] | Wastewater Utility System [Member]
Business Acquisition [Line Items]
Number of customers | customer			7,600
Total purchase price			$ 41,250
Water And Wastewater Utility Systems [Member] | Customer Rate Credits [Member]
Business Acquisition [Line Items]
Revenues from contracts with customers															18,924	$ 4,080
Natural Gas Utility [Member]
Business Acquisition [Line Items]
Customer rate credits							18,924								18,924							18,924
Natural Gas Utility [Member] | Peoples Gas Acquisition [Member]
Business Acquisition [Line Items]
Goodwill		$ 2,277,447
Natural Gas Utility [Member] | Customer Rate Credits [Member]
Business Acquisition [Line Items]
Customer rate credits							$ 23,004								$ 23,004							$ 23,004