Income taxes - Rate reconciliation (Details)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation from U.S. federal statutory tax rate to effective income taxes rate
Federal statutory rate	21.00%	21.00%	34.00%	34.00%
State and local		19.70%	9.60%	2.20%
Foreign rate differential		(0.50%)	(0.70%)	(0.40%)
Stock options		(47.20%)	0.40%	(1.50%)
Excess tax benefits from stock-based compensation		106.40%	34.30%	2.80%
Non-controlling interests		5.50%	1.10%	0.60%
Valuation allowance		(90.70%)	(83.60%)	(38.90%)
Uncertain tax positions		2.30%	0.60%	(0.20%)
Effect of U.S. tax reform law changes		0.00%	14.70%	0.00%
Convertible Notes		94.90%	0.00%	0.00%
Other		(5.90%)	(0.40%)	(0.20%)
Income taxes		105.50%	10.00%	(1.60%)