Quarterly report pursuant to Section 13 or 15(d)

Benefit Plans - Fair Value of Options Granted to Employees and Members of Our Board of Directors (Details)

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Benefit Plans - Fair Value of Options Granted to Employees and Members of Our Board of Directors (Details) - Employee Stock Option [Member]
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Fair value of options granted to employees and members of our board of directors        
Risk-free interest rate 1.30% 1.60% 1.30% 1.30%
Expected life of options — years 3 years 9 months 29 days 4 years 9 months 11 days 3 years 9 months 29 days 4 years 1 month 10 days
Expected stock price volatility 26.00% 40.00% 26.00% 36.00%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%