Quarterly Financial Data (unaudited) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues:
Grant revenue									$ 0us-gaap_RevenueFromGrants	$ 0us-gaap_RevenueFromGrants	$ 147us-gaap_RevenueFromGrants
Total revenues											147us-gaap_Revenues
Operating expenses:
Research and development	15,653	16,208	18,761	17,583	19,981	17,623	17,876	16,380	68,205	71,860	49,412
General and administrative	4,241us-gaap_GeneralAndAdministrativeExpense	3,241us-gaap_GeneralAndAdministrativeExpense	2,940us-gaap_GeneralAndAdministrativeExpense	5,324us-gaap_GeneralAndAdministrativeExpense	3,463us-gaap_GeneralAndAdministrativeExpense	4,171us-gaap_GeneralAndAdministrativeExpense	4,187us-gaap_GeneralAndAdministrativeExpense	3,878us-gaap_GeneralAndAdministrativeExpense	15,746us-gaap_GeneralAndAdministrativeExpense	15,699us-gaap_GeneralAndAdministrativeExpense	11,676us-gaap_GeneralAndAdministrativeExpense
Total operating expenses	19,894us-gaap_OperatingExpenses	19,449us-gaap_OperatingExpenses	21,701us-gaap_OperatingExpenses	22,907us-gaap_OperatingExpenses	23,444us-gaap_OperatingExpenses	21,794us-gaap_OperatingExpenses	22,063us-gaap_OperatingExpenses	20,258us-gaap_OperatingExpenses	83,951us-gaap_OperatingExpenses	87,559us-gaap_OperatingExpenses	61,088us-gaap_OperatingExpenses
Loss from operations	(19,894)us-gaap_OperatingIncomeLoss	(19,449)us-gaap_OperatingIncomeLoss	(21,701)us-gaap_OperatingIncomeLoss	(22,907)us-gaap_OperatingIncomeLoss	(23,444)us-gaap_OperatingIncomeLoss	(21,794)us-gaap_OperatingIncomeLoss	(22,063)us-gaap_OperatingIncomeLoss	(20,258)us-gaap_OperatingIncomeLoss	(83,951)us-gaap_OperatingIncomeLoss	(87,559)us-gaap_OperatingIncomeLoss	(60,941)us-gaap_OperatingIncomeLoss
Interest expense, net	(458)	(517)	(585)	(650)	(718)	(721)	(724)	(470)	(2,210)	(2,633)	(1,849)
Net loss	$ (20,352)	$ (19,966)	$ (22,286)	$ (23,557)	$ (24,162)	$ (22,515)	$ (22,787)	$ (20,728)	$ (86,161)	$ (90,192)	$ (62,790)
Basic and diluted net loss per common share (in dollars per share)	$ (0.19)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.19)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.24)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.28)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.31)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.33)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.33)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.30)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.87)us-gaap_EarningsPerShareBasicAndDiluted	$ (1.27)us-gaap_EarningsPerShareBasicAndDiluted	$ (1.06)us-gaap_EarningsPerShareBasicAndDiluted
Basic and diluted weighted average number of common shares outstanding	108,366,504	105,774,949	94,046,278	85,438,127	76,769,199	69,047,161	69,034,823	68,991,371	98,489,470	70,976,705	59,411,476

Name:	snta_CollaborationRevenuesAbstract
Namespace Prefix:	snta_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_CostsAndExpensesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

No definition available.

+ Details

Name:	us-gaap_EarningsPerShareBasicAndDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate total of expenses of managing and administering the affairs of an entity, including affiliates of the reporting entity, which are not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

+ Details

Name:	us-gaap_GeneralAndAdministrativeExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net amount of nonoperating interest income (expense).

+ References

No definition available.

+ Details

Name:	us-gaap_InterestIncomeExpenseNonoperatingNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

+ Details

Name:	us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingExpenses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The costs incurred in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, excluding in-process research and development acquired in a business combination consummated during the period. Excludes software research and development, which has a separate concept.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 730

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Research and Development

-URI http://asc.fasb.org/extlink&oid=6523717

+ Details

Name:	us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Revenue earned during the period from non-repayable sum of money awarded to an entity to carry out a specific purpose as provided in grant agreements.

+ References

+ Details

Name:	us-gaap_RevenueFromGrants
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ Details

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS).

+ References

No definition available.

+ Details

Name:	us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration