Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Letters of Credit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 65.0	$ 51.0
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 0.0	$ 0.0
Letter of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Period	1 year