Income Taxes Income Taxes - Provision (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Current tax expense (benefit)	$ 4.8	$ 33.1	$ 31.2
Deferred income taxes, net	207.3	180.0	192.5
Investment tax credit, net	(1.1)	(1.1)	(1.1)
Total income tax expense	$ 211.0	$ 212.0	$ 222.6