Income Taxes - Net Deferred Tax Liability (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Depreciation & cost recovery timing differences	$ 593.7	$ 902.4
Regulatory assets recoverable through future rates	7.9	17.6
Alternative minimum tax carryforward	(12.2)	(29.3)
Employee benefit obligations	(9.3)	(8.1)
Employee benefit obligations	(4.1)	(3.2)
Net operating loss & other carryforwards (net of valuation allowances)	(12.2)	0.0
Regulatory liabilities to be settled through future rates	(116.2)	(15.9)
Impairments	(0.6)	(2.5)
Deferred fuel costs-net	16.2	25.9
Other-net	28.1	18.8
Net noncurrent deferred tax liability	$ 491.3	$ 905.7