Borrowings and Other Financing Instruments, Amended Credit Agreement (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	6 Months Ended
	Jul. 31, 2012 Period	Jun. 30, 2012	Dec. 31, 2012
Amended Credit Agreement [Abstract]
Amended term (in years)	5 years
Original term (in years)		4 years
Minimum borrowing margin based on long-term credit ratings (in hundredths)	0.875%	1.00%
Maximum borrowing margin based on long-term credit ratings (in hundredths)	1.75%	2.00%
Minimum commitment fees calculated on unused portion of lines of credit (in hundredths)	0.075%	0.10%
Maximum commitment fees calculated on unused portion of lines of credit (in hundredths)	0.275%	0.35%
Number of additional periods the revolving termination date can be extended, subject to majority bank group approval	1
Term of each additional period the revolving termination date can be extended, subject to majority bank group approval (in years)	1 year
Maximum debt-to-total capitalization ratio allowed (in hundredths)			65.00%
Debt-to-total capitalization ratio (in hundredths)			50.00%
Minimum threshold percentage of subsidiary assets to consolidated assets required to initiate cross default provisions (in hundredths)			15.00%
Minimum amount of indebtedness in default to initiate cross default provisions			$ 75