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	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	$ 2.9
Wtd avg period of compensation cost not yet recognized	1 year 1 month 21 days
Purchases under ESPP (in shares)	259,000	198,000
Exercised (in dollars per share)	$ 25.51	$ 22.74