Annual and transition report of foreign private issuers pursuant to Section 13 or 15(d)

Equity (Schedule of Stock Options Assumptions) (Details)

v3.8.0.1
Equity (Schedule of Stock Options Assumptions) (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 0.00%    
Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate, Minimum 1.80% 1.10% 1.60%
Risk-free interest rate, Maximum 2.20% 1.50% 1.90%
Expected option term (years)     6 years
Expected share price volatility, Minimum 52.60% 53.60% 50.10%
Expected share price volatility, Maximum 54.00% 56.10% 53.50%
Dividend yield
Weighted average grant date fair value $ 11.10 $ 12.36 $ 15.49
Minimum [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected option term (years) 5 years 1 month 6 days 5 years 2 months 12 days  
Maximum [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected option term (years) 6 years 6 years