Borrowings and Other Financing Instruments - Money Pool (Details) - USD ($)	3 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Short-term Debt [Line Items]
Short-term Debt	$ 0		$ 250,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000		100,000,000
Short-term Debt	63,000,000		0
Average amount outstanding	6,000,000	$ 43,000,000
Maximum amount outstanding	$ 94,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis	0.02%	0.54%
Weighted average interest rate at period end	0.02%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 500,000,000		500,000,000
Short-term Debt	0		$ 250,000,000
Average amount outstanding	0	$ 44,000,000
Maximum amount outstanding	$ 0	$ 250,000,000
Weighted average interest rate, computed on a daily basis		1.11%
Weighted average interest rate at period end			0.29%