Operations and summary of significant accounting policies - Adjustment For Excess And Obsolete Inventories (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Inventory Valuation Reserve [Roll Forward]
Balance at Beginning of Period	$ 15,385	$ 16,447	$ 12,639
Provisions	6,046	7,870	7,130
Write-Offs	5,942	8,932	3,322
Balance at End of Period	$ 15,489	$ 15,385	$ 16,447