Annual report pursuant to Section 13 and 15(d)

Employee Benefits and Share-Based Compensation - Fair value assumptions of Share Option Awards (Details)

v3.8.0.1
Employee Benefits and Share-Based Compensation - Fair value assumptions of Share Option Awards (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Time Based Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield   0.00% 0.00%
Expected stock price volatility, minimum   30.36% 32.32%
Expected stock price volatility, maximum   33.01% 45.33%
Risk-free interest rate, minimum   1.20% 1.34%
Risk-free interest rate, maximum   1.48% 1.92%
Expected term   6 years  
Time Based Options | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term     6 years
Time Based Options | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term     6 years 6 months
Performance Based Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 0.00% 0.00% 0.00%
Expected share price volatility 25.97%    
Expected stock price volatility, minimum   25.97% 29.31%
Expected stock price volatility, maximum   30.21% 29.86%
Risk-free interest rate, minimum   1.01%  
Risk-free interest rate, maximum   1.93%  
Risk-free interest rate 1.81%   1.76%
Expected term 4 years 2 months 12 days    
Performance Based Options | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term   4 years 4 months 17 days 4 years 10 months 17 days
Performance Based Options | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term   5 years 1 month 17 days 5 years 4 months 17 days
Market Based Share Option Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield     0.00%
Expected share price volatility     30.00%
Risk-free interest rate     1.34%
Expected term     Mid-point from vesting to assumed options expiration