Stockholders' equity - Changes in Common Stock and Treasury Stock (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	65.9	54.2	53.4
Debt exchange (in shares)		10.6
Stock-based compensation (in shares)	0.9	1.1	0.8
Ending Balance (in shares)	66.8	65.9	54.2
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	3.5	3.1	2.9
Debt exchange (in shares)		0.0
Stock-based compensation (in shares)	0.3	0.4	0.2
Ending Balance (in shares)	3.8	3.5	3.1