Borrowings and Other Financing Instruments, Short-Term Borrowings (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	$ 42.0	$ 42.0	$ 0.0
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100.0	100.0	100.0	$ 100.0
Amount outstanding at period end	0.0	0.0	0.0	0.0
Average amount outstanding	14.0	29.0	13.0	28.0
Maximum amount outstanding	$ 74.0	$ 100.0	$ 100.0	$ 100.0
Weighted average interest rate, computed on a daily basis (percentage)	2.13%	1.96%	1.12%	0.67%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400.0	$ 400.0	$ 400.0	$ 400.0
Amount outstanding at period end	42.0	42.0	0.0	50.0
Average amount outstanding	20.0	30.0	69.0	43.0
Maximum amount outstanding	$ 100.0	$ 144.0	$ 176.0	$ 140.0
Weighted average interest rate, computed on a daily basis (percentage)	2.45%	2.27%	1.13%	0.67%
Weighted average interest rate at period end (percentage)	2.80%	2.80%		0.95%