Derivative Instruments - Cash Collateral and Forward Interest Swap (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
cash collateral and interest rate swap
Derivative, Collateral, Right to Reclaim Cash	$ 35.7	$ 11.2
Aggregate fair value of derivative instruments with credit risk-related contingent features that were in a liability position	20.9	0.0
Collateral that would have been required	19.6	$ 0.0
Amount of hedged item deferred into accumulated other comprehensive income	19.0
Amount of accumulated other comprehensive income reclassified into income	0.4
Amount of accumulated other comprehensive income to be reclassified into income over the next twelve months	1.3
Total senior notes issued in June 2015
cash collateral and interest rate swap
Issuance of long-term debt related to the acquisition of Integrys	$ 1,200.0