Borrowings and Other Financing Instruments - Short-Term Borrowings (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Short-term Debt [Line Items]
Short-term debt	$ 0		$ 136,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000		250,000,000
Short-term debt	0		57,000,000
Average amount outstanding	33,000,000		59,000,000
Maximum amount outstanding	$ 243,000,000		$ 250,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.08%		0.60%
Short-term Debt, Weighted Average Interest Rate, at Point in Time			0.07%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 700,000,000		$ 700,000,000
Short-term debt	0		136,000,000
Average amount outstanding	99,000,000		30,000,000
Maximum amount outstanding	$ 322,000,000		$ 230,000,000
Weighted average interest rate, computed on a daily basis (percentage)		0.19%	1.59%
Short-term Debt, Weighted Average Interest Rate, at Point in Time			0.20%