Stock-Based Compensation Expense - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Compensation cost not yet recognized		$ 20.7
Compensation cost not yet recognized, period of recognition		3 years 3 months
Options granted in period (in shares)	20,000,000	692,002	1,300,000