Stockholders' equity - Changes in Common Stock and Treasury Stock (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	66.8	65.9	54.2
Debt exchange (in shares)	4.6		10.6
Stock-based compensation (in shares)	1.1	0.9	1.1
Ending Balance (in shares)	72.5	66.8	65.9
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	3.8	3.5	3.1
Debt exchange (in shares)	1.2		0.0
Stock-based compensation (in shares)	0.3	0.3	0.4
Ending Balance (in shares)	5.3	3.8	3.5