Acquisitions (Pro Forma Summary) (Details) - Peoples Gas Acquisition [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Business Acquisition [Line Items]
Operating revenues	$ 348,647	$ 341,636	$ 1,269,768	$ 1,292,057
Net income	$ 55,732	$ 75,461	$ 264,785	$ 198,066