Revenue - Concentration of Credit Risk (Details) - Customer concentration	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
MACI and Carticel | Revenue Concentration
CONCENTRATION
Concentration risk		55.00%	24.00%	25.00%
MACI and Carticel | Revenue Concentration | Maximum
CONCENTRATION
Concentration risk	10.00%
MACI and Carticel | Accounts Receivable Concentration
CONCENTRATION
Concentration risk					46.00%
MACI and Carticel | Accounts Receivable Concentration | Maximum
CONCENTRATION
Concentration risk			10.00%
Epicel | Revenue Concentration
CONCENTRATION
Concentration risk		10.00%		13.00%
Epicel | Revenue Concentration | Maximum
CONCENTRATION
Concentration risk	10.00%		10.00%
Epicel | Accounts Receivable Concentration | Maximum
CONCENTRATION
Concentration risk			10.00%		10.00%