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Share-Based Compensation - Valuation Assumptions (Details) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Options      
Weighted-average assumptions      
Risk-free interest rate, minimum (as a percent) 2.30% 2.00% 1.10%
Risk-free interest rate, maximum (as a percent) 3.00% 2.10% 1.90%
Expected term (in years) 6 years 6 years 6 years
Expected volatility, minimum (as a percent) 53.00% 54.00% 53.00%
Expected volatility, maximum (as a percent) 54.00% 56.00% 73.00%
Weighted-average estimated fair value (in dollars per share) $ 14.32 $ 17.29 $ 13.28
2013 ESPP      
Weighted-average assumptions      
Risk-free interest rate, minimum (as a percent) 2.10% 0.90% 0.40%
Risk-free interest rate, maximum (as a percent) 2.80% 1.70% 1.00%
Expected volatility, minimum (as a percent) 42.00% 41.00% 54.00%
Expected volatility, maximum (as a percent) 53.00% 56.00% 65.00%
Weighted-average estimated fair value (in dollars per share) $ 9.13 $ 7.09 $ 9.63
2013 ESPP | Minimum      
Weighted-average assumptions      
Expected term (in years) 6 months 6 months 6 months
2013 ESPP | Maximum      
Weighted-average assumptions      
Expected term (in years) 2 years 2 years 2 years