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, $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchases under ESPP (in shares)	465,696	420,000
Exercised (in dollars per share)	$ 25.78	$ 23.23
Unrecognized compensation cost	$ 1.6
Weighted average period of compensation cost not yet recognized (in years)	1 year 6 months 22 days