Statements of Consolidated Comprehensive Income Statements of Consolidated Comprehensive Income (Parenthetical) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Increase Decrease Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax	$ (1.7)	$ 0.7	$ 0.8
Other Comprehensive Income Increase Decrease In Unrealized Gain Loss On Derivatives Arising During Period Tax	2.1	1.1	7.6
Unrealized Gain (Loss) on Derivatives	1.0	1.4	15.4
Change In Unrecognized Pension Benefit And OPEB Costs Tax	$ 4.2	$ 3.7	$ 0.4