Fair Value Disclosures (Reconciliation Of Level 3 Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2012 Other Derivatives [Member]	Sep. 30, 2011 Other Derivatives [Member]	Sep. 30, 2012 Other Derivatives [Member]	Mar. 31, 2012 Other Derivatives [Member]	Mar. 31, 2011 Other Derivatives [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Beginning balance	$ 0.2			$ 0.3	$ (0.3)	$ (0.8)
Total gains (losses) (unrealized/realized) Included in regulatory assets/liabilities	(0.7)	0.1	0.4	(0.5)
Purchases	(1.1)		(0.2)
Settlements	1.2		0.2
Ending balance	(0.4)	(0.2)	(0.4)	(0.2)	(0.3)	(0.8)
Change in unrealized gains/(losses) relating to instruments still held	$ (1.1)	$ 0.2	$ (1.1)	$ 0.2