Income Taxes - Summary of income tax expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Current tax expense	$ (127.5)	$ 111.8	$ 72.7
Deferred income taxes, net	300.1	274.4	498.7
Investment tax credit, net	(2.8)	(2.7)	(4.9)
Total income tax expense	$ 169.8	$ 383.5	$ 566.5