v2.4.0.8

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013 Collaborator	Sep. 30, 2012	Sep. 30, 2013 Segment Collaborator	Sep. 30, 2012	Dec. 31, 2012 Segment	Dec. 31, 2011
Organization And Significant Accounting Policies [Line Items]
Research and development commitments with third parties	$ 2,786		$ 2,786		$ 3,164	$ 6,220
Research and development commitments with third parties not incurred	1,267		1,267		1,431	1,665
Maturity period of liquid investment			3 months		3 months
Cash equivalent investments in highly liquid money market accounts	56,693		56,693		9,384	18,833
Short-term investments	136,672		136,672		260	258
Fair value of financial assets measured at fair value on a recurring basis	325,348		325,348		83,376	39,097
Unrealized appreciation (depreciation) in the fair value of the Company's equity securities	27,339	(3,940)	5,704	12,031	(6,290)	(2,675)
Carrying value of investment in VIEs	11,540		11,540		5,726
Percentage of recognized income tax positions			50.00%		50.00%
Number of segment			1		1
Investments gross unrealized losses	9		9
Number of collaborators identified as VIEs	2		2
Equity Securities [Member]
Organization And Significant Accounting Policies [Line Items]
Fair value of financial assets measured at fair value on a recurring basis	107,567		107,567		83,116	39,097
Ziopharm [Member]
Organization And Significant Accounting Policies [Line Items]
Company's ownership percentage of Ziopharm	16.20%		16.20%		16.30%	11.50%
Ziopharm [Member] | Equity Securities [Member]
Organization And Significant Accounting Policies [Line Items]
Fair value of financial assets measured at fair value on a recurring basis	53,321		53,321		56,298	35,162
Unrealized appreciation (depreciation) in the fair value of the Company's equity securities	24,766	(4,948)	(2,977)	8,773	(7,194)	(4,924)
Certificates of Deposit [Member]
Organization And Significant Accounting Policies [Line Items]
Short-term investments					$ 260	$ 258
Minimum [Member]
Organization And Significant Accounting Policies [Line Items]
Maturity of certificate of deposits					3 Months
Minimum [Member] | Patents and Related Technologies [Member]
Organization And Significant Accounting Policies [Line Items]
Period of estimated useful lives			7 years		7 years
Minimum [Member] | Leasehold improvements [Member]
Organization And Significant Accounting Policies [Line Items]
Life of the asset			1 year		1 year
Maximum [Member]
Organization And Significant Accounting Policies [Line Items]
Maturity of certificate of deposits					1 Year
Maximum [Member] | Patents and Related Technologies [Member]
Organization And Significant Accounting Policies [Line Items]
Period of estimated useful lives			14 years		14 years
Maximum [Member] | Leasehold improvements [Member]
Organization And Significant Accounting Policies [Line Items]
Life of the asset			4 years		4 years

X

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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Period Type:	instant

X

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

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X

- Definition

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 30

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14172-108612

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X

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Balance Type:	na
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X

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

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X

- Definition

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

+ References

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
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X

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
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X

- Definition

Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph g

-Article 7

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of accumulated pre-tax unrealized loss before deducting pre-tax unrealized gain on investments in available-for-sale debt securities impacting investments.

+ References

No definition available.

+ Details

Name:	xon_AvailableforsaleDebtSecuritiesGrossUnrealizedLossAccumulatedInInvestments
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- Definition

Highly liquid investment maximum maturity period.

+ References

No definition available.

+ Details

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Namespace Prefix:	xon_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Income tax benefit recognition criteria percentage threshold.

+ References

No definition available.

+ Details

Name:	xon_IncomeTaxBenefitRecognitionCriteriaPercentageThreshold
Namespace Prefix:	xon_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of collaborators identified as vie.

+ References

No definition available.

+ Details

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Namespace Prefix:	xon_
Data Type:	xbrli:integerItemType
Balance Type:	na
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X

- Details

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X

- Definition

Research and development commitments.

+ References

No definition available.

+ Details

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Balance Type:	credit
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X

- Definition

Research and development commitments not yet incurred.

+ References

No definition available.

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Namespace Prefix:	xon_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- Definition

Short term investments maturity period.

+ References

No definition available.

+ Details

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Namespace Prefix:	xon_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The carrying value of the entity's investment in identified Variable Interest Entities (VIEs).

+ References

No definition available.

+ Details

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Namespace Prefix:	xon_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant