Income taxes - Income Tax Expense (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income tax (benefit) expense	$ 100,000	$ (3,900,000)	$ 4,826,000	$ (4,428,000)	$ 1,359,000
Effective tax rate	0.30%	(4.30%)	(7.80%)	23.20%	(1.30%)
Current Federal Tax Expense (Benefit)			$ (164,000)	$ (52,000)	$ (2,821,000)
Current State and Local Tax Expense (Benefit)			84,000	48,000	175,000
Current Foreign Tax Expense (Benefit)	$ 1,500,000	$ 300,000	4,956,000	(4,391,000)	4,394,000
Current Income Tax Expense (Benefit)			4,876,000	(4,395,000)	1,748,000
Deferred Federal Income Tax Expense (Benefit)			0	0	248,000
Deferred Foreign Income Tax Expense (Benefit)			(50,000)	(33,000)	(637,000)
Deferred Income Tax Expense (Benefit)			(50,000)	$ (33,000)	$ (389,000)
Deferred Tax Assets, Tax Credit Carryforwards, Other	$ 16,800,000		$ 16,800,000