Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity and Capital Stock - assumptions (Details)

v3.8.0.1
Shareholders' Equity and Capital Stock - assumptions (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 57.00% 57.00% 57.00%
Risk-free interest rate, Maximum 1.60% 1.00% 0.70%
Maximum | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected award life (in years) 3 years 8 months 27 days 3 years 8 months 19 days 3 years 8 months 1 day
Forfeiture rate (options) 6.00% 5.60% 5.00%
Maximum | Restricted Stock Units (RSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Forfeiture rate (RSUs) 6.10% 6.20% 8.30%
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility, Minimum 56.00% 57.00% 55.00%
Risk-free interest rate, Minimum 1.00% 1.00% 0.50%
Minimum | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected award life (in years) 3 years 8 months 23 days 3 years 8 months 19 days 3 years 7 months 6 days
Forfeiture rate (options) 5.30% 5.60% 4.90%
Minimum | Restricted Stock Units (RSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Forfeiture rate (RSUs) 6.10% 6.20% 7.20%