Employee Benefits and Share-Based Compensation - Employee Stock Purchase Plan (ESPP) Activity (Details) - 1997 Employee stock purchase plan - Employee stock purchase plan (ESPP) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchases under ESPP (in shares)	210	289
Exercised (in dollars per share)	$ 40.20	$ 26.30
Unrecognized compensation cost	$ 3.9
Weighted average period of compensation cost not yet recognized (in years)	1 year 6 months 4 days