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	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	$ 1.9
Weighted average period of compensation cost not yet recognized (in years)	1 year 4 months 6 days
Purchases under ESPP (in shares)	259	198
Exercised (in dollars per share)	$ 25.51	$ 22.74