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Fair Value-Based Measurements and Weighted-Average Assumptions (Detail) - $ / shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average fair value per share	$ 5.19	$ 10.39
Risk-free interest rate	2.50%	2.10%
Expected life (in years)	1 year 2 months 12 days	1 year 3 months 18 days
Volatility	0.80%	1.10%
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average fair value per share	$ 6.85	$ 10.84
Risk-free interest rate	2.50%	2.60%
Expected life (in years)	4 years 6 months	4 years 6 months
Volatility	0.90%	0.80%

X

- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(ii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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