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Income taxes - Reconciliation of Federal statutory income rate to effective income tax rate (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Reconciliation from the U.S. Federal statutory income rate of 34.0% to the effective income tax rate      
Federal statutory rate 34.00% 34.00% 34.00%
State tax 0.38% 0.24% 9.20%
Permanent differences 0.42% 2.49% 54.30%
Foreign tax rates at rates different from U.S. rates (11.90%) (22.06%) (82.70%)
Valuation allowance (85.02%) 6.05% 50.20%
Total tax (benefit) provision (62.12%) 20.72% 65.00%
Uncertain tax positions $ 0.0 $ 0.0