Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Amended Credit Agreements (Details) (Credit Facility, USD $)			1 Months Ended		1 Months Ended
	Sep. 30, 2014		Oct. 31, 2014 Subsequent Event	Oct. 14, 2014 Subsequent Event	Oct. 31, 2014 Subsequent Event Minimum	Oct. 31, 2014 Subsequent Event Maximum	Oct. 31, 2014 Subsequent Event Eurodollar Minimum	Oct. 31, 2014 Subsequent Event Eurodollar Maximum
Line of Credit Facility [Line Items]
Amended Credit Agreements, Commencement Date			Oct. 14, 2014
Amended Term (in years)			5 years
Borrowing Limit	$ 700,000,000	[1]		$ 700,000,000
Borrowing Margin Based On Long-Term Credit Ratings (percentage)							0.875%	1.75%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage					0.075%	0.275%
Number Of Additional Periods The Revolving Termination Date On The Credit Facility Can Be Extended, Subject To Majority Bank Group Approval			2
Term Of Each Additional Period The Revolving Termination Date Can Be Extended, Subject To Majority Bank Group Approval (in years)			1 year

[1] Credit facility has been amended to expire in October 2019.