Summary of Significant Accounting Policies Nature of Operations (Details) customer in Millions	Dec. 31, 2020 customer
Electric
Product Information
Number of customers	1.6
Natural gas
Product Information
Number of customers	3.0
ATC
Product Information
Equity method investment, ownership interest (as a percent)	60.00%
ATC Holdco
Product Information
Equity method investment, ownership interest (as a percent)	75.00%

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant