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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Jun. 30, 2011 Elimination of Medicare Part D [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	8.8	10.3	20.4	23.4		(2.9)
Non-Current Regulatory Assets	$ 340.3		$ 340.3		$ 340.3	$ 5.0