Rate Matters (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Rate Proceedings [Abstract]
Authorized percentage increase (decrease) for electric	4.30%	2.30%	3.30%	(0.74%)
Authorized rate increase (decrease) for electric	$ 15.7	$ 8.0	$ 11.9	$ (2.7)
Authorized percentage increase (decrease) for gas	0.30%	1.00%	(0.74%)
Authorized rate increase (decrease) for gas	0.6	1.9	(1.5)
Fuel rules bandwidth		2.00%	2.00%
Authorized return on common stock		10.30%	10.40%
Utility common equity		58.10%	55.30%
Fuel Rules [Abstract]
Electric fuel deferred costs upper threshold		102.00%
Electric fuel deferred costs lower threshold		98.00%
Authorized electric fuel costs credit interest				10.80%
Authorized electric fuel costs refund			0.3	5.5
Revenue reduction from an electric fuel costs refund or credit				$ 4.6