v3.8.0.1
Income taxes - Rate reconciliation (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation from U.S. federal statutory tax rate to effective income taxes rate      
Federal statutory rate 34.00% 34.00% 34.00%
State and local 9.60% 2.20% 4.10%
Foreign rate differential (0.70%) (0.40%) (0.50%)
Stock options 0.40% (1.50%) (1.50%)
Excess tax benefits from stock-based compensation 34.30% 2.80% 0.00%
Non-controlling interests 1.10% 0.60% 0.50%
Valuation allowance (83.60%) (38.90%) (39.00%)
Uncertain tax positions 0.60% (0.20%) (0.10%)
Effect of U.S. tax reform law changes 14.70% 0.00% 0.00%
Other (0.40%) (0.20%) 0.30%
Income taxes 10.00% (1.60%) (2.20%)