Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) (USD $) In Millions, unless otherwise specified			1 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2012 Bonds [Member] Series Due Oct. 1, 2042	Dec. 31, 2013 Bonds [Member] Series Due Oct. 1, 2042
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 100.0
Debt Instrument, Interest Rate, Stated Percentage			3.70%	3.70%
Debt Instrument, Maturity Date			Oct. 01, 2042	Oct. 01, 2042
Long-term Debt, Maturities, Repayments of Principal in Year Five	150.0
Deferred Finance Costs, Noncurrent, Net	3.5	3.6
Maximum annual dividends that can be paid if equity capitalization ratio condition is not met	31.2
Minimum calendar year average equity to total capitalization ratio authorized by state commission	52.50%
Calendar year average equity to total capitalization ratio	52.80%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 17.1