Income Taxes - Summary of income tax expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Current tax expense (benefit)	$ 73.4	$ (56.2)	$ 81.5
Deferred income tax expense (benefit), net	(128.9)	0.1	110.6
Investment tax credit, net	(2.3)	(0.8)	(0.9)
Total income tax expense (benefit)	$ (57.8)	$ (56.9)	$ 191.2