Regulatory Matters - The Patient Protection and Affordable Care Act of 2010 (PPACA) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			3 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Jun. 30, 2011 MPUC Approval of Deferral of PPACA [Member]	Dec. 31, 2010 Elimination of Medicare Part D [Member]
Unusual or Infrequent Item [Line Items]
Reversal of Previously Recorded Tax Benefit							$ 4.0
Deferred Income Tax Benefit	(10.3)	(12.8)	(23.4)	(24.6)		2.9
Non-Current Regulatory Assets	$ 334.6		$ 334.6		$ 345.9	$ 5.0