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Significant Accounting Policies (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Share-based Compensation
Expected term	6 years 3 months	6 years 3 months
Level 1
Fair Value Measurements
Carrying value of cash held in money market fund	$ 182.2		$ 207.9
Level 2
Fair Value Measurements
Assets measured at fair value levels 2 or 3	0.0		0.0
Liabilities measured at fair value levels 2 or 3	0.0		0.0
Level 3
Fair Value Measurements
Assets measured at fair value levels 2 or 3	0.0		0.0
Liabilities measured at fair value levels 2 or 3	$ 0.0		$ 0.0
Maximum
Share-based Compensation
Expiration period	10 years

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