Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Goodwill
Accumulated impairment losses	$ 0.0
Goodwill impairment loss	0.0
Changes to goodwill balances by segment
Goodwill balance as of January 1	3,053.5	$ 3,046.2
Acquisition of Bluewater	0.0	7.3
Adjustment to Bluewater purchase price allocation	(0.7)	0.0
Goodwill balance as of December 31	3,052.8	3,053.5
Wisconsin
Changes to goodwill balances by segment
Goodwill balance as of January 1	2,104.3	2,104.3
Acquisition of Bluewater	0.0	0.0
Adjustment to Bluewater purchase price allocation	0.0	0.0
Goodwill balance as of December 31	2,104.3	2,104.3
Illinois
Changes to goodwill balances by segment
Goodwill balance as of January 1	758.7	758.7
Acquisition of Bluewater	0.0	0.0
Adjustment to Bluewater purchase price allocation	0.0	0.0
Goodwill balance as of December 31	758.7	758.7
Other States
Changes to goodwill balances by segment
Goodwill balance as of January 1	183.2	183.2
Acquisition of Bluewater	0.0	0.0
Adjustment to Bluewater purchase price allocation	0.0	0.0
Goodwill balance as of December 31	183.2	183.2
Non-Utility Energy Infrastructure
Changes to goodwill balances by segment
Goodwill balance as of January 1	7.3	0.0
Acquisition of Bluewater	0.0	7.3
Adjustment to Bluewater purchase price allocation	(0.7)	0.0
Goodwill balance as of December 31	$ 6.6	$ 7.3

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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