Income taxes, Effective tax rate reconciliation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Tax Rate Reconciliation [Abstract]
Federal statutory tax rate	21.00%	21.00%	34.00%
Valuation allowance and tax accruals	14.80%	1.00%	1.60%
State income taxes, net of federal income taxes	12.00%	1.20%	1.30%
Business meals and entertainment	5.40%	0.40%	0.40%
Miscellaneous permanent items	1.40%	0.30%	(0.90%)
Uncertain tax positions	1.00%	(0.00%)	(0.10%)
Stock option cancellations	0.80%	0.00%	1.70%
U.S. corporate tax rate change	0.00%	0.00%	19.40%
Foreign-derived intangible income deduction	(5.40%)	(1.50%)	(0.00%)
Stock award excess tax benefit	(8.40%)	(1.50%)	(1.40%)
R&D credit	(83.20%)	(4.90%)	(3.30%)
Other	0.00%	0.10%	(0.10%)
Effective tax rate	(40.60%)	16.10%	52.60%