Financing Activities (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Montana and South Dakota first mortgage bonds, interest rate 4.15%, maturing in 2042 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 90	$ 90
Debt instrument, interest rate	4.15%	4.15%
Debt instrument, maturity date		Aug. 10, 2042
Montana and South Dakota first mortgage bonds, interest rate 4.30%, maturing in 2052 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	60	60
Debt instrument, interest rate	4.30%	4.30%
Debt instrument, maturity date		Aug. 10, 2052
Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date		Aug. 10, 2012
Common Stock [Member]
Debt Instrument [Line Items]
Equity Distribution Agreement Date		Apr. 01, 2012
Common stock, aggregate gross sales price		100
Stock issued during period, shares, new issues	128,131
Average share price	$ 36.50	$ 36.50
Stock issued during period, value, new issues	4.6
Additional Paid-in Capital [Member]
Debt Instrument [Line Items]
Adjustments to additional paid in capital, issuance costs	$ 0.1