Acquisitions (Narrative) (Details)		1 Months Ended					3 Months Ended				4 Months Ended	6 Months Ended		12 Months Ended
	Mar. 16, 2020 USD ($) customer	Jun. 30, 2020 USD ($) customer	Jan. 31, 2020 USD ($) customer	Dec. 31, 2019 USD ($) customer	Nov. 30, 2019 USD ($) customer	Sep. 30, 2019 USD ($) customer item	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Apr. 24, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) customer item	Mar. 13, 2020 USD ($)	Oct. 22, 2018 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 2,333,535,000		$ 63,822,000			$ 2,333,535,000					$ 2,333,535,000		$ 63,822,000
Fair value adjustment									$ (11,040,000)				$ 23,742,000
Operating revenues							384,468,000		218,892,000			640,053,000	420,024,000
Net income							74,629,000	$ 51,781,000	54,903,000	$ 16,924,000		126,410,000	71,827,000
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														2,393
Total purchase price														$ 9,437,000
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					20,628,000		16,464,000			20,628,000	16,464,000
Acquisition-related costs, pre-tax		25,197,000					25,197,000		21,406,000			$ 25,197,000	21,406,000
Adjustments to acquisition financing													35,812,000
Adjustments to acquisition financing, Pre-tax													46,129,000
Adjustments to debt													7,084,000
Adjustments to debt, pre-tax													9,962,000
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	2,468,946,000
Goodwill	$ 2,269,437,000
Fair value adjustment													11,040,000
Transaction-related expenses							25,397,000		$ 12,744,000			25,397,000	$ 19,390,000
Operating revenues												188,181,000
Net income												$ 31,753,000
Decrease in goodwill							$ (18,240,000)
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