Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of income tax expense
Current tax expense (benefit)	$ 72.7	$ 15.1	$ 33.6
Deferred income taxes	498.7	420.4	329.2
Investment tax credit, net	(4.9)	(1.7)	(1.1)
Total income tax expense, amount	566.5	433.8	361.7
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Balance, January 1	9.5	7.2
Acquired legacy Integrys tax assets	0.0	(3.6)
Additions for tax positions of prior years	6.7	0.3
Additions based on tax positions related to the current year	1.1	0.2
Reductions for tax positions of prior years	(1.0)	(1.1)
Reductions due to statute of limitations	(1.8)	0.0
Settlements during the period	0.0	(0.7)
Balance, December 31	14.5	9.5	7.2
Income Taxes
Deferred tax assets, uncertainty in income taxes	6.6	6.2
Net amount of unrecognized tax benefits having impact on the effective tax rate for continuing operations	7.9	2.2
Accrued interest in the consolidated income statements	0.2	0.0	0.3
Accrued penalties in the consolidated income statements	0.0	0.0	$ 0.0
Accrued interest on the consolidated balance sheets	0.8	0.7
Accrued penalties on the consolidated balance sheets	$ 0.0	$ 0.1