Acquisitions (Preliminary Purchase Price Allocation) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Mar. 15, 2021	Dec. 31, 2020	Mar. 16, 2020
Business Acquisition [Line Items]
Goodwill	$ 2,340,836			$ 2,324,547
Peoples Gas Acquisition [Member]
Business Acquisition [Line Items]
Property, plant and equipment, net					$ 2,476,551
Current assets					233,334
Regulatory assets					264,458
Goodwill		$ 16,400			2,277,447
Other long-term assets					75,071
Total assets acquired					5,326,861
Current portion of long-term debt					5,136
Loans payable					181,000
Other current liabilities					185,920
Long-term debt					999,460
Deferred income taxes					193,125
Regulatory liabilities					129,418
Other long-term liabilities					167,458
Total liabilities assumed					1,861,517
Net assets acquired					3,465,344
Peoples Gas Acquisition [Member] | Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Property, plant and equipment, net					2,476,551
Current assets					242,531
Regulatory assets					286,751
Goodwill					2,261,047
Other long-term assets					75,071
Total assets acquired					5,341,951
Current portion of long-term debt					5,136
Loans payable					181,000
Other current liabilities					186,120
Long-term debt					999,460
Deferred income taxes					213,647
Regulatory liabilities					123,029
Other long-term liabilities					168,215
Total liabilities assumed					1,876,607
Net assets acquired					$ 3,465,344
Peoples Gas Acquisition [Member] | Revision of Prior Period, Adjustment [Member]
Business Acquisition [Line Items]
Current assets			$ (9,197)
Regulatory assets			(22,293)
Goodwill			16,400
Total assets acquired			(15,090)
Other current liabilities			(200)
Deferred income taxes			(20,522)
Regulatory liabilities			6,389
Other long-term liabilities			(757)
Total liabilities assumed			$ (15,090)