Fair Value Measurements, Derivatives (Details) - Derivative [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair value of derivatives classified as Level 3 in the fair value hierarchy
Beginning Balance	$ 3.3	$ 1.7	$ 7.0	$ 3.5
Realized and unrealized gains (losses)	0.0	0.0	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	3.9	15.6	3.9	15.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0.0	0.0	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(3.6)	(3.2)	(7.3)	(5.0)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset and Liability Acquisition of Integrys	(1.3)	0.0	(1.3)	0.0
Transfers in and/or out of Level 3	0.0	0.0	0.0	0.0
Ending Balance	$ 2.3	$ 14.1	$ 2.3	$ 14.1