Acquisitions (Narrative) (Details)		1 Months Ended					3 Months Ended		12 Months Ended
	Mar. 16, 2020 USD ($) customer	Jan. 31, 2020 USD ($) customer	Dec. 31, 2019 USD ($) customer	Nov. 30, 2019 USD ($) customer	Sep. 30, 2019 USD ($) customer item	Nov. 30, 2018 USD ($) customer	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) item customer	Mar. 13, 2020 USD ($)	Oct. 22, 2018 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 63,822,000				$ 2,351,526,000		$ 63,822,000
Fair value adjustment								$ 34,782,000
Operating revenues							255,585,000	201,132,000
Net income							51,781,000	16,924,000
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	16,464,000
Acquisition-related costs, pre-tax							$ 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
Customer rate credits							23,000,000
Property, plant and equipment, net	2,468,946,000
Goodwill	$ 2,287,677,000
Fair value adjustment								34,782,000
Transaction-related expenses							25,397,000	$ 6,646,000
Operating revenues							38,544,000
Net income							$ 13,398,000
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				2,106		4,950
Total purchase price				$ 29,500,000		$ 21,000,000
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