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Common Stock, Preferred Stock and Equity Plans - Employee Stock Purchase Plan (Details) - ESPP share issuances
12 Months Ended
Dec. 31, 2017
period
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Offering period (in months) 12 months
Number of purchase periods | period 2
Duration of purchase period 6 months
Eligible employee purchase price percentage of fair value 85.00%
Number of shares authorized (shares) 616,256
Number of shares (shares) 275,000
Average price paid per share (usd per share) | $ / shares $ 80.71