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Significant Accounting Policies (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
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	$ 227.4		$ 121.9
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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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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