Financing (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended		12 Months Ended
	Jan. 31, 2011 Y State	Jan. 31, 2010 quarter	Sep. 30, 2011	Jan. 28, 2011	Jan. 26, 2010
Financing
Shelf registration statement, amount				$ 400
Number of states to which electric service is provided	4
Maximum amount of securities to be issued in the form of first mortgage bonds	250
Effective period to use proceeds under this shelf to fund capital expenditures, refinancings of existing debt or general corporate needs (in years)	3
Unsecured revolving credit facility					150
Maximum percentage of total indebtedness to total capitalization under the financial covenants of the credit facility agreement (as a percent)		62.50%
Minimum interest coverage ratio based on EBITDA under the financial covenants of the credit facility agreement		2
Number of trailing fiscal quarters used for calculating the interest coverage ratio based on EBITDA under the financial covenants of the credit facility agreement		4
Total indebtedness as a percent of total capitalization (as a percent)			50.50%
EBITDA coverage ratio over interest charges			5.25
Aggregate amount of default on other indebtedness subject to cross-default under the terms of the credit facility.		10
Amount of credit facility used to back up outstanding commercial paper borrowings			$ 4.0