Income Taxes - Summary of income tax expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Current tax expense (benefit)	$ 49.2	$ (37.9)	$ (127.5)
Deferred income taxes, net	182.2	167.7	300.1
Investment tax credit, net	(3.5)	(4.8)	(2.8)
Total income tax expense	$ 227.9	$ 125.0	$ 169.8