Goodwill and Other Intangible Assets - Goodwill (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Jun. 30, 2015	Sep. 30, 2015
Goodwill
Goodwill impairment loss		$ 0.0
Changes to the carrying amount of goodwill
Goodwill balance at December 31, 2014		441.9
Acquisition of Integrys	$ 2,947.2
Goodwill balance at September 30, 2015		3,389.1
Integrys
Changes to the carrying amount of goodwill
Acquisition of Integrys		$ 2,947.2