Stockholders' equity - Changes in Common Stock and Treasury Stock (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	73.3	72.5	66.8
Debt exchange (in shares)			4.6
Stock-based compensation (in shares)	0.7	0.8	1.1
Ending Balance (in shares)	74.0	73.3	72.5
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	5.5	5.3	3.8
Debt exchange (in shares)			1.2
Stock-based compensation (in shares)	0.1	0.2	0.3
Ending Balance (in shares)	5.6	5.5	5.3