Regulatory Matters - The Patient Protection and Affordable Care Act of 2010 (PPACA) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unusual or Infrequent Item [Line Items]
Deferred Income Tax Expense (Benefit)		$ 37.5	$ 35.8	$ 66.0
Non-Current Regulatory Asset		340.3	345.9
PPACA Elimination of Medicare Part D [Member]
Unusual or Infrequent Item [Line Items]
Reversal of Previously Recorded Tax Benefit		0	0	4.0
Deferred Income Tax Expense (Benefit)	2.9	0	(2.9)	0
Non-Current Regulatory Asset		$ 5.0