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)	$ (37.9)	$ (127.5)	$ 111.8
Deferred income taxes, net	167.7	300.1	274.4
Investment tax credit, net	(4.8)	(2.8)	(2.7)
Total income tax expense	$ 125.0	$ 169.8	$ 383.5