Income taxes - Component of Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income taxes
Expected tax expense (benefit), at U.S. federal statutory income tax rate of 21%	$ (1,348)	$ 8,241	$ 12,776
Nontaxable dividends received from Kronos	(5,621)	(5,621)	(5,325)
U.S. state income taxes and other, net	(32)	165	28
Income tax expense (benefit)	$ (7,001)	$ 2,785	$ 7,479