Stock-Based Compensation Expense - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Compensation cost not yet recognized	$ 22.4		$ 22.4
Compensation cost not yet recognized, period of recognition			3 years 5 months 16 days
Options granted in period (in shares)	200,000	1,280,000	692,002	1,280,000