Investment in equity method investee - Selected Financial Data from the Investee (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Equity Method Investments [Line Items]
Net sales	$ 426,186		$ 410,219		$ 757,062	$ 702,077
Gross profit	105,579		85,814		164,321	127,619
Income from operations	51,030		26,216		55,038	16,978
Net income	27,115	$ (14,066)	2,954	$ (25,039)	13,049	(22,085)
CP&P Joint Venture
Schedule of Equity Method Investments [Line Items]
Net sales	36,032		37,353		70,790	65,602
Gross profit	10,933		11,721		21,470	19,675
Income from operations	6,259		6,840		11,879	10,020
Net income	$ 6,209		$ 6,776		$ 11,767	$ 9,893