Borrowings and Other Financing Instruments, Credit Facility (Details) (Credit Facility, USD $)	1 Months Ended	12 Months Ended
	Oct. 31, 2014	Dec. 31, 2014		Dec. 31, 2013
Line of Credit Facility [Line Items]
Line Of Credit Facility Maximum Amount Credit Facility May Be Increased		$ 50,000,000
Line Of Credit Facility Maximum Debt To Total Capitalization Ratio Allowed		65.00%
Line Of Credit Facility Debt To Total Capitalization Ratio		47.00%		49.00%
Line of Credit Facility, Initiation Date	Oct. 14, 2014
Credit Facility		400,000,000	[1]
Drawn		67,000,000	[2]
Available		333,000,000
Direct advances on the credit outstanding		0		0
Debt Instrument, Term	5 years
Original Credit Facility, Maximum Borrowing Limit	300,000,000
Line Of Credit Facility Minimum Threshhold Percentage Of Subsidiary Assets To Consolidated Assets Required To Initiate Cross Default Provisions		15.00%
Line of Credit Facility, Minimum Amount of Indebtedness in Default to Initiate Cross Default Provisions		$ 75,000,000
Number Of Additional Periods Revolving Termination Date Can Be Extended Subject To Majority Bank Group Approval		2
Term Of Each Additional Period Revolving Termination Date Can Be Extended Subject To Majority Bank Group Approval		1 year
Minimum
Line of Credit Facility [Line Items]
Line Of Credit Facility Minimum Commitment Fees Calculated On Unused Portion Of Lines Of Credit		0.075%
Maximum
Line of Credit Facility [Line Items]
Line Of Credit Facility Maximum Commitment Fees Calculated On Unused Portion Of Lines Of Credit		0.275%
Eurodollar | Minimum
Line of Credit Facility [Line Items]
Line Of Credit Facility Minimum Borrowing Margin Based On Long Term Credit Ratings		0.875%
Eurodollar | Maximum
Line of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Margin Based On Long Term Credit Ratings		1.75%

[1] These credit facilities have been amended to extend the maturity to October 2019.

[2] Includes outstanding commercial paper and letters of credit.