Borrowings and Other Financing Instruments, Credit Facility (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Debt Disclosure [Abstract]
Credit agreement term (in years)	5 years
Commercial Paper Available For Issuance	$ 700,000,000	[1]
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
Line Of Credit Facility Maximum Amount Credit Facility May Be Increased	100,000,000
Line Of Credit Facility Maximum Debt To Total Capitalization Ratio Allowed	65.00%
Line Of Credit Facility Debt To Total Capitalization Ratio	45.00%
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
Line Of Credit Facility Minimum Borrowing Margin Based On Long Term Credit Ratings	0.875%
Line Of Credit Facility Maximum Borrowing Margin Based On Long Term Credit Ratings	1.75%
Line Of Credit Facility Minimum Commitment Fees Calculated On Unused Portion Of Lines Of Credit	0.075%
Line Of Credit Facility Maximum Commitment Fees Calculated On Unused Portion Of Lines Of Credit	0.275%
Credit Facility [Abstract]
Drawn	6,400,000	[2]
Available	693,600,000
Credit facility bank borrowings outstanding	$ 0		$ 0

[1] Credit facility expires in July 2017.

[2] Includes outstanding letters of credit.