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Equity Award Plan Activities - Fair Value Assumptions - ESPP (Details) - ESPP
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 1 year 2 months 20 days 1 year 4 months 24 days 1 year 4 months 24 days
Risk-free interest rate 0.60% 0.30% 0.30%
Expected volatility 31.80% 34.80% 36.30%
Dividend rate 0.00% 0.00% 0.00%