Stock Incentive Plans - Components of Restricted Stock Awards (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted stock awards (in shares) \| shares	1,975,862
Unrecognized compensation expense	$ 12,099
Intrinsic value	$ 27,306
Weighted average remaining life (in years)	1 year 9 months 30 days
Service- Based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted stock awards (in shares) \| shares	1,062,902
Unrecognized compensation expense	$ 7,037
Intrinsic value	$ 14,689
Weighted average remaining life (in years)	1 year 11 months 5 days
Performance- Based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted stock awards (in shares) \| shares	912,960
Unrecognized compensation expense	$ 5,062
Intrinsic value	$ 12,617
Weighted average remaining life (in years)	1 year 8 months 18 days

Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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