Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Equity Offerings (Details) - USD ($) shares in Millions, $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Oct. 26, 2018	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Class of Stock [Line Items]
Stock to be Issued under the Dividend Reinvestment Program, Amount		$ 75.0
Proceeds from Issuance of Common Stock				$ 203.0	$ 0.0
Share Issuance Program
Class of Stock [Line Items]
Common Stock Authorized Value Amount		$ 300.0
Common Stock Issued During Period through At-The-Market Offering, Shares			4.2
Transaction fees			$ 1.7
Share Issuance Program | Subsequent Event
Class of Stock [Line Items]
Common Stock Issued During Period through At-The-Market Offering, Shares	0.5
Proceeds from Issuance of Common Stock	$ 25.5
Transaction fees	$ 0.2
Share Issuance Program | At-the-Market Offering
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock			$ 199.3