Income tax provision (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Reconciliation of effective tax rate
Income tax at federal statutory rate	21.00%	21.00%
Goodwill impairment charges (Note 8)	0.00%	46.80%
State income tax, net of federal income tax benefit	3.10%	2.10%
Non-deductible acquisition costs	2.80%	0.00%
Non-deductible executive compensation	1.70%	2.20%
Foreign tax rate differences	1.20%	4.30%
Tax impact of share-based compensation	0.80%	8.50%
Change in unrecognized tax benefits, including interest and penalties	0.40%	(3.30%)
Research and development tax credit	(0.80%)	(3.70%)
Payables and receivables for prior year tax returns	(0.30%)	3.20%
Non-taxable income from employee life insurance policies	(0.30%)	(1.10%)
Return to provision adjustments	(0.10%)	(2.60%)
Change in valuation allowances	0.00%	0.90%
Other	0.20%	1.80%
Effective tax rate	29.70%	80.10%