Fair value measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Available-for-sale securities [Line Items]
Investments used to satisfy nonqualified benefit plans obligations	$ 62.7	$ 66.2	$ 83.5
Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans	2.9		$ 2.9
Investment withdrawal, reduction in cost basis	$ 9.0	$ 20.0