Acquisitions (Narrative) (Details)		1 Months Ended							3 Months Ended							4 Months Ended	9 Months Ended		12 Months Ended
	Mar. 16, 2020 USD ($) customer	Oct. 31, 2020 USD ($) customer	Sep. 30, 2020 USD ($) customer	Jun. 30, 2020 USD ($) customer	Jan. 31, 2020 USD ($) customer	Dec. 31, 2019 USD ($) customer	Nov. 30, 2019 USD ($) customer	Sep. 30, 2019 USD ($) item customer	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Apr. 24, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) customer item	Mar. 13, 2020 USD ($)	Oct. 22, 2018 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 2,333,627,000			$ 63,822,000				$ 2,333,627,000							$ 2,333,627,000		$ 63,822,000
Fair value adjustment																		$ 23,742,000
Operating revenues										348,647,000			$ 243,626,000				988,700,000	663,650,000
Net income										55,732,000	$ 74,629,000	$ 51,781,000	88,489,000	$ 54,903,000	$ 16,924,000		182,142,000	160,316,000
Interest Rate Swap [Member]
Business Acquisition [Line Items]
Debt instrument, issued																$ 900,000,000
Water And Wastewater Utility Systems [Member]
Business Acquisition [Line Items]
Number of acquisitions | item																			7
Number of customers | customer			11,000																2,393
Total purchase price			$ 53,000,000																$ 9,437,000
Goodwill						$ 2,478,000													2,478,000
Peoples Gas Acquisition [Member]
Business Acquisition [Line Items]
Number of customers | customer	747,000
Total purchase price	$ 3,465,344,000
Indebtedness	1,101,091,000																			$ 1,101,091,000
short-term debt	181,000,000																			181,000,000
Outstanding debt	$ 920,091,000																			$ 920,091,000
Measurement period	1 year
Fair value of debt	$ 84,569,000
Acquisition-related costs			20,628,000					$ 16,464,000		20,628,000			16,464,000				20,628,000	16,464,000
Acquisition-related costs, pre-tax			25,197,000					$ 21,406,000		25,197,000			21,406,000				$ 25,197,000	21,406,000
Adjustments to acquisition financing																		39,567,000
Adjustments to acquisition financing, Pre-tax																		50,883,000
Adjustments to debt																		7,971,000
Adjustments to debt, pre-tax																		11,210,000
Elimination of management fee																		885,000
Elimination of management fee, pre-tax																		1,245,000
Pipeline term																	7 years
Pipeline cost			120,000,000							120,000,000							$ 120,000,000
Customer rate credits			$ 23,000,000							23,000,000							23,000,000
Property, plant and equipment, net	2,476,551,000
Goodwill	2,267,310,000
Transaction-related expenses												$ 25,397,000	$ 2,496,000					$ 21,886,000
Operating revenues																	280,300,000
Net income																	$ 6,131,000
Decrease in goodwill										(2,127,000)
Peoples Gas Acquisition [Member] | Interest Rate Swap [Member]
Business Acquisition [Line Items]
Fair value adjustment																$ 23,742,000
Peoples Gas Acquisition [Member] | Bridge Commitment [Member]
Business Acquisition [Line Items]
Debt instrument, issued																					$ 5,100,000,000
Pennsylvania [Member] | Wastewater Utility System [Member]
Business Acquisition [Line Items]
Number of customers | customer				4,947			2,106
Total purchase price				$ 21,000,000			$ 29,500,000
Cheltenham Township [Member] | Wastewater Utility System [Member]
Business Acquisition [Line Items]
Number of customers | customer						9,887
Total purchase price						$ 50,250,000
Property, plant and equipment, net						44,440,000													44,440,000
Goodwill						$ 5,810,000													$ 5,810,000
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								$ 276,500,000
Ohio [Member] | Water And Wastewater Utility Systems [Member]
Business Acquisition [Line Items]
Number of customers | customer					3,126
Total purchase price					$ 7,500,000
Subsequent Event [Member] | Water And Wastewater Utility Systems [Member]
Business Acquisition [Line Items]
Number of customers | customer		256
Total purchase price		$ 4,815,000
Customer Rate Credits [Member] | Scenario, Forecast [Member] | Peoples Gas Acquisition [Member]
Business Acquisition [Line Items]
Revenues from contracts with customers									$ 23,000,000
Water And Wastewater Utility Systems [Member] | Customer Rate Credits [Member]
Business Acquisition [Line Items]
Revenues from contracts with customers										4,080,000
Water And Wastewater Utility Systems [Member] | Customer Rate Credits [Member] | Peoples Gas Acquisition [Member]
Business Acquisition [Line Items]
Revenues from contracts with customers										$ 4,080,000
Natural Gas Utility [Member] | Peoples Gas Acquisition [Member]
Business Acquisition [Line Items]
Goodwill	$ 2,267,310,000
Natural Gas Utility [Member] | Customer Rate Credits [Member] | Scenario, Forecast [Member]
Business Acquisition [Line Items]
Revenues from contracts with customers									$ 18,920,000