Borrowings and Other Financing Instruments - Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2021	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	2,000,000	59,000,000
Maximum amount outstanding	$ 38,000,000	$ 250,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.05%	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	0	30,000,000
Maximum amount outstanding	$ 0	$ 230,000,000
Weighted average interest rate, computed on a daily basis (percentage)		1.59%
Short-term Debt, Weighted Average Interest Rate, at Point in Time		0.20%