Borrowings and Other Financing Instruments - Money Pool (Details) - Money Pool - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Short-term Debt [Line Items]
Borrowing limit	$ 100,000,000	$ 100,000,000
Amount outstanding at period end	0	0
Average amount outstanding	17,000,000	43,000,000
Maximum amount outstanding	$ 100,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis	0.07%	0.54%