XML 68 R57.htm IDEA: XBRL DOCUMENT v3.8.0.1
Employee Benefits and Share-Based Compensation - Employee Stock Purchase Plan (ESPP) Activity (Details) - Nineteen Ninety Seven Employee Stock Purchase Plan - Employee Stock Purchase Plan - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Purchases under ESPP (in shares) 289 259
Exercised (in dollars per share) $ 26.30 $ 25.51
Unrecognized compensation cost   $ 6.5
Weighted average period of compensation cost not yet recognized (in years) 1 year 4 months 21 days