Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding at period end	$ 51.0	$ 51.0	$ 11.0
Notes payable to affiliates	0.6	0.6	0.5
payable to affiliates [Member]
Short-term Debt [Line Items]
Notes payable to affiliates	$ 0.6	$ 0.6	$ 0.5
Weighted average interest rate at period end (percentage)	2.89%	2.89%	1.73%
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	51.0	51.0	11.0	60.0
Average amount outstanding	51.0	28.0	52.0	15.0
Maximum amount outstanding	$ 103.0	$ 103.0	$ 129.0	$ 64.0
Weighted average interest rate, computed on a daily basis (percentage)	2.56%	2.31%	1.23%	0.69%
Weighted average interest rate at period end (percentage)	2.89%	2.89%	1.73%	0.95%