Income taxes - Component of Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		18 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023
Income taxes
Expected tax expense (benefit), at U.S. federal statutory income tax rate of 21%	$ (2,077)	$ 7,873	$ (3,026)	$ 12,475
U.S. federal statutory income tax rate					21.00%
Rate differences on equity in earnings (losses) of Kronos, net of dividends	(5,146)	(1,449)	(2,841)	(3,363)
U.S. state income taxes and other, net	(77)	11	(12)	28
Income tax expense	$ (7,300)	$ 6,435	$ (5,879)	$ 9,140