Marketable securities - Schedule of Marketable Securities (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Marketable securities [Line Items]
Market value, current	$ 13.8	$ 56.1
Market value, noncurrent	5.3	4.8
Current Assets
Marketable securities [Line Items]
Market value, current	13.8	56.1
Cost or amortized cost	13.8	56.1
Unrealized loss, net	0.0	0.0
Non Current Assets
Marketable securities [Line Items]
Market value, noncurrent	5.3	4.8
Cost or amortized cost	5.3	5.0
Unrealized loss, net	$ 0.0	$ (0.2)