Earnings or loss per share - reconciliation (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings (numerator)
Basic	$ 147.9	$ 30.9
Dilutive effect of stock-based awards	0.0	0.0
Diluted	$ 147.9	$ 30.9
Weighted average shares outstanding (denominator)
Basic	581.1	573.7
Dilutive effect of stock-based awards	8.0	7.6
Diluted	589.1	581.3
Earnings per share
Basic	$ 0.25	$ 0.05
Diluted	$ 0.25	$ 0.05