Other Financial Statement Items - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended
	Jun. 29, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 25, 2020	Sep. 01, 2020	Jul. 24, 2020	Jun. 30, 2020	May 01, 2020	Apr. 09, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Supplemental Balance Sheet Information [Line Items]
Allowance for doubtful accounts, deferred payment arrangement		$ 94,000,000	$ 73,000,000	$ 94,000,000	$ 73,000,000				$ 85,000,000			$ 69,000,000	$ 73,000,000	$ 62,000,000
Provision for doubtful accounts, accounts receivable		19,000,000	24,000,000	60,000,000	71,000,000
Prepaid other taxes		179,000,000		179,000,000								123,000,000
Notes payable		998,000,000		998,000,000								560,000,000
South Dakota Wind Farm | Disposed of by Sale
Supplemental Balance Sheet Information [Line Items]
Percentage of ownership transferred								85.00%
Consideration from disposition								$ 236,000,000
Assets held for sale from disposition		196,000,000		$ 196,000,000
2020 Credit Facility | Revolving Credit Facility
Supplemental Balance Sheet Information [Line Items]
Line of credit facility, maximum borrowing capacity	$ 500,000,000
Credit facility, commitment fee percentage	0.20%
Debt extension term	1 year			1 year
Debt extension cost (in basis points)	0.75%
2020 Credit Facility | Min. | Revolving Credit Facility
Supplemental Balance Sheet Information [Line Items]
Credit facility, commitment fee percentage	0.15%
2020 Credit Facility | Max. | Revolving Credit Facility
Supplemental Balance Sheet Information [Line Items]
Credit facility, commitment fee percentage	0.30%
Commercial Paper
Supplemental Balance Sheet Information [Line Items]
Notes payable		999,000,000		$ 999,000,000								562,000,000
Senior Notes | Unsecured Notes Maturing in 2025
Supplemental Balance Sheet Information [Line Items]
Face amount of debt											$ 750,000,000
Interest rate											3.20%
Senior Notes | Pollution Control Bonds Due from 2026 to 2029
Supplemental Balance Sheet Information [Line Items]
Face amount of debt										$ 200,000,000
Senior Notes | Pollution Control Bonds Due from 2026 to 2029 | Min.
Supplemental Balance Sheet Information [Line Items]
Interest rate										1.40%
Senior Notes | Pollution Control Bonds Due from 2026 to 2029 | Max.
Supplemental Balance Sheet Information [Line Items]
Interest rate										1.61%
Senior Notes | Unsecured Notes Maturing in 2050
Supplemental Balance Sheet Information [Line Items]
Face amount of debt							$ 25,000,000
Interest rate							3.68%
Senior Notes | Unsecured Notes Maturing in 2030
Supplemental Balance Sheet Information [Line Items]
Face amount of debt						$ 200,000,000
Interest rate						1.95%
Deferred Payment Arrangements
Supplemental Balance Sheet Information [Line Items]
Accounts receivable		77,000,000		77,000,000								65,000,000
Allowance for doubtful accounts, deferred payment arrangement		37,000,000		37,000,000								$ 33,000,000
Provision for doubtful accounts, accounts receivable		$ 3,000,000	$ 1,000,000	$ 4,000,000	$ 3,000,000