Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 65.0	$ 65.0	$ 51.0
Notes payable to affiliates	0.0	0.0	0.6
payable to affiliates [Member]
Short-term Debt [Line Items]
Notes payable to affiliates	0.0	0.0	$ 0.6
Weighted average interest rate at period end (percentage)			2.89%
Commercial Paper [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	150.0	150.0	$ 150.0	$ 150.0
Amount outstanding at period end	65.0	65.0	51.0	11.0
Average amount outstanding	72.0	51.0	28.0	52.0
Maximum amount outstanding	$ 93.0	$ 93.0	$ 103.0	$ 129.0
Weighted average interest rate, computed on a daily basis (percentage)	1.98%	2.38%	2.31%	1.23%
Weighted average interest rate at period end (percentage)	1.97%	1.97%	2.89%	1.73%