Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Goodwill
Accumulated impairment losses	$ 0.0
Goodwill impairment loss	0.0
Changes to goodwill balances by segment
Goodwill balance as of January 1	3,023.5	$ 441.9
Adjustment to Integrys purchase price allocation	22.7	0.0
Acquisition of Integrys	0.0	2,581.6
Goodwill balance as of December 31	3,046.2	3,023.5
Wisconsin
Changes to goodwill balances by segment
Goodwill balance as of January 1	2,109.5	441.9
Adjustment to Integrys purchase price allocation	(5.2)	0.0
Acquisition of Integrys	0.0	1,667.6
Goodwill balance as of December 31	2,104.3	2,109.5
Illinois
Changes to goodwill balances by segment
Goodwill balance as of January 1	731.2	0.0
Adjustment to Integrys purchase price allocation	27.5	0.0
Acquisition of Integrys	0.0	731.2
Goodwill balance as of December 31	758.7	731.2
Other States
Changes to goodwill balances by segment
Goodwill balance as of January 1	182.8	0.0
Adjustment to Integrys purchase price allocation	0.4	0.0
Acquisition of Integrys	0.0	182.8
Goodwill balance as of December 31	$ 183.2	$ 182.8