Stock-Based Compensation Plans - Additional Information (Detail) shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense \| $	$ 6.6
Weighted average period to recognize expense	1 year 4 months 24 days
1998 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserve for issuance or grant	33.0
2001 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserve for issuance or grant	50.6
Other Than Options [Member] \| 2001 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserve for issuance or grant	14.0
Director Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserve for issuance or grant	1.7
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense \| $	$ 126.6
Weighted average period to recognize expense	1 year 6 months 20 days
Performance-Based Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense \| $	$ 31.0
Weighted average period to recognize expense	1 year
Employee Stock Purchase Plan (ESPP) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserve for issuance or grant	6.0
Discount allowed to employees on purchase of shares under ESPP plan	5.00%

Discount allowed to employees on purchase of shares under ESPP plan.

No definition available.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ 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 (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

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 maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ 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 (a)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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