Summary of Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation expense expressed as a percentage of average depreciable property	3.10%	2.90%	2.60%
Cash and Cash Equivalents [Abstract]
Maximum number of months of remaining maturity at time of purchase to consider investments in certain instruments as cash equivalents	three months
Accounts and Financing Receivable, after Allowance for Credit Loss, Current and Noncurrent [Abstract]
Accounts Receivable, Allowance for Credit Loss, Current	$ 29	$ 21
Alternative Revenue Programs [Abstract]
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs	24 months
Number of DSM Products, Commercial and Industrial Customers	20
Number of DSM Products, Residential and Low-income Customers	23
DSM Measurements	1,000
Public Utilities, Inventory [Line Items]
Inventories	$ 189	192
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	63	63
Public Utilities, Inventory, Fuel [Member]
Public Utilities, Inventory [Line Items]
Inventories	73	77
Public Utilities, Inventory, Natural Gas [Member]
Public Utilities, Inventory [Line Items]
Inventories	$ 53	$ 52