Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Tax Cuts and Jobs Act of 2017 tax benefit	$ 45.3
Investment tax credits	1.2	$ 1.6
State net operating loss carryforwards	4.1	3.2
Charitable contributions carryforwards	12.2	0.0
Valuation allowance	10.1	8.3
Uncertain tax positions [Abstract]
Net liability for unrecognized tax benefits inclusive of interest, penalties and net of secondary impacts which are a component of the deferred income taxes and are benefits	1.3	$ 1.2
Utility Group
Operating Loss Carryforwards [Line Items]
Tax Cuts and Jobs Act of 2017 tax benefit	23.2
Excess federal deferred income taxes from Tax Cuts and Jobs Act of 2017	333.4
Nonutility Group
Operating Loss Carryforwards [Line Items]
Tax Cuts and Jobs Act of 2017 tax benefit	22.3
Corporate and Other
Operating Loss Carryforwards [Line Items]
Tax Cuts and Jobs Act of 2017 tax benefit	$ (0.2)