Borrowings and Other Financing Instruments - Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Money Pool
Short-term Debt [Line Items]
Borrowing limit	$ 250,000,000	$ 250,000,000
Short-term Debt	0	39,000,000
Average amount outstanding	94,000,000	7,000,000
Maximum amount outstanding	$ 250,000,000	$ 50,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.92%	2.29%
Short-term Debt, Weighted Average Interest Rate, at Point in Time		1.63%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 700,000,000	$ 700,000,000
Short-term Debt	0	0
Average amount outstanding	24,000,000	154,000,000
Maximum amount outstanding	$ 166,000,000	$ 432,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.71%	2.67%