Borrowings and Other Financing Instruments (Details) (USD $)	1 Months Ended	6 Months Ended	9 Months Ended			3 Months Ended	12 Months Ended
	Jul. 31, 2012	Jun. 30, 2012	Sep. 30, 2012		Dec. 31, 2011	Sep. 30, 2012 Commercial Paper [Member]	Dec. 31, 2011 Commercial Paper [Member]	Sep. 30, 2012 Clearwater Investments, Inc. [Member] Notes Payable [Member]	Dec. 31, 2011 Clearwater Investments, Inc. [Member] Notes Payable [Member]
Short-term Debt [Line Items]
Borrowing limit						$ 150,000,000	$ 150,000,000
Amount outstanding at period end			99,000,000		66,000,000	99,000,000	66,000,000
Average amount outstanding						88,000,000	24,000,000
Maximum amount outstanding						116,000,000	70,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)						0.38%	0.37%
Weighted average interest rate at period end (in hundredths)						0.38%	0.46%	0.33%	0.46%
Notes payable to affiliates			550,000		500,000			600,000	500,000
Credit Facilities [Abstract]
Credit facility			150,000,000
Drawn			99,000,000	[1]
Available			51,000,000
Credit facility bank borrowings outstanding			0		0
Amended Credit Agreement [Abstract]
Amended term (in years)	5Y
Original term (in years)		4Y
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)	1Y
Letters of Credit [Abstract]
Terms of letters of credit (in years)			1Y
Letters of credit outstanding under credit facilities			$ 0		$ 0

[1] Includes outstanding commercial paper.