The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section 50
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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Number of options outstanding, including both vested and non-vested options.

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-Publisher FASB
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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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-Publisher FASB
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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.

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No definition available.

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The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Share-Based Compensation - Schedule of Information with Respect to Outstanding and Exercisable Stock Options (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number of Options Outstanding	2,868,053	3,085,888	2,657,723	3,781,964
Options Outstanding, Weighted Average Exercise Price	$ 9.86	$ 11.18	$ 14.27	$ 12.88
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	6 years 10 days
Options Exercisable, Number Exercisable	1,788,346
Options Exercisable, Weighted Average Exercise Price	$ 12.28
Range of Exercise Prices $2.65 - $3.93 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	2.65
Range of Exercise Prices, Maximum	$ 3.93
Options Outstanding, Number of Options Outstanding	347,248
Options Outstanding, Weighted Average Exercise Price	$ 3.29
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	6 years 3 months 10 days
Options Exercisable, Number Exercisable	347,248
Options Exercisable, Weighted Average Exercise Price	$ 3.29
Range of Exercise Prices $4.07 - $4.15 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	4.07
Range of Exercise Prices, Maximum	$ 4.15
Options Outstanding, Number of Options Outstanding	307,380
Options Outstanding, Weighted Average Exercise Price	$ 4.14
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	7 years 7 months 24 days
Options Exercisable, Number Exercisable	153,690
Options Exercisable, Weighted Average Exercise Price	$ 4.14
Range of Exercise Prices $4.49 - $4.49 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	4.49
Range of Exercise Prices, Maximum	$ 4.49
Options Outstanding, Number of Options Outstanding	683,104
Options Outstanding, Weighted Average Exercise Price	$ 4.49
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	8 years 2 months 4 days
Options Exercisable, Number Exercisable	170,776
Options Exercisable, Weighted Average Exercise Price	$ 4.49
Range of Exercise Prices $5.00 - $6.51 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	5.00
Range of Exercise Prices, Maximum	$ 6.51
Options Outstanding, Number of Options Outstanding	346,012
Options Outstanding, Weighted Average Exercise Price	$ 6.00
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	6 years 11 months 1 day
Options Exercisable, Number Exercisable	282,094
Options Exercisable, Weighted Average Exercise Price	$ 6.05
Range of Exercise Prices $7.22 - $8.30 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	7.22
Range of Exercise Prices, Maximum	$ 8.30
Options Outstanding, Number of Options Outstanding	324,832
Options Outstanding, Weighted Average Exercise Price	$ 8.07
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	8 years 6 months 21 days
Options Exercisable, Number Exercisable	59,126
Options Exercisable, Weighted Average Exercise Price	$ 7.31
Range of Exercise Prices $8.63 - $18.64 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	8.63
Range of Exercise Prices, Maximum	$ 18.64
Options Outstanding, Number of Options Outstanding	330,929
Options Outstanding, Weighted Average Exercise Price	$ 14.26
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	2 years 10 months 20 days
Options Exercisable, Number Exercisable	246,864
Options Exercisable, Weighted Average Exercise Price	$ 15.74
Range of Exercise Prices $21.80 - $22.13 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	21.80
Range of Exercise Prices, Maximum	$ 22.13
Options Outstanding, Number of Options Outstanding	298,556
Options Outstanding, Weighted Average Exercise Price	$ 22.02
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	2 years 7 days
Options Exercisable, Number Exercisable	298,556
Options Exercisable, Weighted Average Exercise Price	$ 22.02
Range of Exercise Prices $26.15 - $26.15 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	26.15
Range of Exercise Prices, Maximum	$ 26.15
Options Outstanding, Number of Options Outstanding	36,144
Options Outstanding, Weighted Average Exercise Price	$ 26.15
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	1 year 1 month 24 days
Options Exercisable, Number Exercisable	36,144
Options Exercisable, Weighted Average Exercise Price	$ 26.15
Range of Exercise Prices $29.02 - $29.02 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	29.02
Range of Exercise Prices, Maximum	$ 29.02
Options Outstanding, Number of Options Outstanding	49,848
Options Outstanding, Weighted Average Exercise Price	$ 29.02
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	1 year 10 months 6 days
Options Exercisable, Number Exercisable	49,848
Options Exercisable, Weighted Average Exercise Price	$ 29.02
Range of Exercise Prices $30.67 - $30.67 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Prices, Minimum	30.67
Range of Exercise Prices, Maximum	$ 30.67
Options Outstanding, Number of Options Outstanding	144,000
Options Outstanding, Weighted Average Exercise Price	$ 30.67
Options Outstanding, Weighted Average Remaining Contractual Term (in Years)	2 years 18 days
Options Exercisable, Number Exercisable	144,000
Options Exercisable, Weighted Average Exercise Price	$ 30.67