Income taxes - Component of Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Income taxes
Expected tax expense, at U.S. federal statutory income tax rate of 21%	$ (2,900)	$ 2,500	$ 9,600	$ 9,100
Rate differences on equity in earnings of Kronos, net of dividends	(2,700)	(1,500)	(6,100)	(3,700)
U.S. state income taxes and other, net	100	100	100	100
Income tax expense	$ (5,576)	$ 1,054	$ 3,564	$ 5,460