Goodwill (Details) - USD ($) $ in Millions		12 Months Ended
Jul. 01, 2019	Dec. 31, 2019	Dec. 31, 2018
Goodwill
Accumulated impairment losses		$ 0.0
Goodwill impairment loss	$ 0.0
Changes to goodwill balances by segment
Goodwill balance as of January 1		3,052.8	$ 3,053.5
Adjustment to Bluewater purchase price allocation		0.0	(0.7)
Goodwill balance as of December 31		3,052.8	3,052.8
Wisconsin
Changes to goodwill balances by segment
Goodwill balance as of January 1		2,104.3	2,104.3
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
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
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		6.6	7.3
Adjustment to Bluewater purchase price allocation		0.0	(0.7)
Goodwill balance as of December 31		$ 6.6	$ 6.6

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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Amount of accumulated impairment loss for 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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