Derivative Instruments (Details 4) - Jun. 30, 2015 - USD ($) $ in Millions	Total
Derivative [Line Items]
Amount of hedged item deferred into accumulated other comprehensive income	$ 19.0
Amount of accumulated other comprehensive income reclassified into income	0.1
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
Derivative [Line Items]
Issuance of long-term debt related to the acquisition of Integrys	$ 1,200.0