Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Dividend and Other Restrictions (Details) $ in Millions	Dec. 31, 2018 USD ($)
Dividend and Other Restrictions [Abstract]
Equity to total capitalization ratio (excluding short-term debt), low end of range	45.00%
Equity to total capitalization ratio (excluding short-term debt), high end of range	55.00%
Equity to total capitalization ratio (excluding short-term debt)	54.40%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 605.7
Capitalization, Short term debt, long term debt and equity	$ 4,700.0