Stock-Based Compensation Expense - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Payment Arrangement [Abstract]
Compensation cost not yet recognized	$ 32.4		$ 32.4
Compensation cost not yet recognized, period of recognition			3 years 3 months 26 days
Options granted in period (in shares)	190,000	200,000	1,100,000	692,002