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, $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchases under ESPP (in shares)	374,000	452,038
Exercised (in dollars per share)	$ 41.44	$ 29.69
Compensation cost	$ 2.6
Weighted average period of compensation cost not yet recognized (in years)	1 year 6 months