Income taxes - Component of Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income taxes
Expected tax expense, at U.S. federal statutory income tax rate of 21%	$ 8,200	$ 12,800	$ 2,900
Nontaxable income	(5,600)	(5,300)	(5,300)
U.S. state income taxes and other, net	200		(100)
Income tax expense	$ 2,785	$ 7,479	$ (2,515)