Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Amended Credit Agreements (Details) (Credit Facility, Subsequent Event, USD $) In Millions, unless otherwise specified	1 Months Ended
	Oct. 31, 2014
Line of Credit Facility [Line Items]
Amended Credit Agreements, Commencement Date	Oct. 14, 2014
Amended Term (in years)	5 years
Amended Credit Facility, Maximum Borrowing Limit	$ 400
Original Credit Facility, Maximum Borrowing Limit	$ 300
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
Minimum
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.075%
Maximum
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.275%
Eurodollar | Minimum
Line of Credit Facility [Line Items]
Borrowing Margin Based On Long-Term Credit Ratings (percentage)	0.875%
Eurodollar | Maximum
Line of Credit Facility [Line Items]
Borrowing Margin Based On Long-Term Credit Ratings (percentage)	1.75%