Derivative Instruments Derivative Instruments (Details 3) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	$ 0.2	$ 2.9
Derviative Instrument by Counterparty, Gross Amount Recognized on Balance Sheet [Member]
Derivative [Line Items]
Derivative Assets	11.9	19.2
Derivative Liabilities	0.3	1.9
Derviative Instrument by Counterparty, Gross Amount Not Offset on Balance Sheet [Member] [Member]
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	0	1.3
Derivative Assets	(0.1)	(0.5)
Derivative Liabilities	(0.1)	(1.8)
Derviative Instrument by Counterparty, Net Amount [Member]
Derivative [Line Items]
Derivative Assets	11.8	18.7
Derivative Liabilities	$ 0.2	$ 0.1