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Income Tax - Reconciliation of Beginning and Ending Amounts of Unrecognized Tax Benefits (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Income Tax Disclosure [Abstract] |  |  |
Balance at beginning of year | $ 255us-gaap_UnrecognizedTaxBenefits | $ 227us-gaap_UnrecognizedTaxBenefits | $ 148us-gaap_UnrecognizedTaxBenefits
Additions based on tax positions related to the current year | 51us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions | 52us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions | 44us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
Additions for tax positions of prior years | 43us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 27us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 39us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Reduction for tax positions of prior years | (2)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions | (48)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions |
Reduction due to ST-Ericsson deconsolidation |  | (8)stm_UnrecognizedTaxBenefitsDecreasesResultingFromDeconsolidation |
Settlements |  |  | (1)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
Prepayment | (5)stm_UnrecognizedTaxBenefitsRecordedInOtherAsset | (1)stm_UnrecognizedTaxBenefitsRecordedInOtherAsset | (6)stm_UnrecognizedTaxBenefitsRecordedInOtherAsset
Foreign currency translation | (29)stm_UnrecognizedTaxBenefitsIncreasesDecreaseResultingFromForeignCurrencyTranslation | 6stm_UnrecognizedTaxBenefitsIncreasesDecreaseResultingFromForeignCurrencyTranslation | 3stm_UnrecognizedTaxBenefitsIncreasesDecreaseResultingFromForeignCurrencyTranslation
Balance at end of year | $ 313us-gaap_UnrecognizedTaxBenefits | $ 255us-gaap_UnrecognizedTaxBenefits | $ 227us-gaap_UnrecognizedTaxBenefits

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- Definition Unrecognized Tax Benefits Decreases Resulting From Deconsolidation + References No definition available. + Details
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- Definition Unrecognized Tax Benefits Increases Decrease Resulting From Foreign currency translation. + References No definition available. + Details
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- Definition Unrecognized Tax Benefits Recorded In Other Asset + References No definition available. + Details
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- Details
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- Definition Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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