XML 71 R51.htm IDEA: XBRL DOCUMENT v3.8.0.1
Employee benefit plans - Fair Value Assumptions for Restricted Stock Units and ESPP (Details) - Employee Stock Purchase Plan Shares [Member]
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (years) 6 months 6 months 6 months
Dividend yield 0.00% 0.00% 0.00%