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v2.4.1.9

Actuarial Assumptions to Determine Benefit Obligations (Detail) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Taiwan |  |  |
Weighted-average assumptions used to determine benefit obligations: |  |  |
Discount rate | 2.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 1.88%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 1.63%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember
Rate of compensation increase | 4.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 4.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 4.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember
Weighted-average assumptions used to determine net projected benefit cost: |  |  |
Discount rate | 2.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 1.88%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 1.63%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember
Expected long-term return on plan assets | 2.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 2.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 1.88%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember
Rate of compensation increase | 4.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 4.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember | 4.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = simo_TaiwanMember
Korea |  |  |
Weighted-average assumptions used to determine benefit obligations: |  |  |
Discount rate | 4.10%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 5.20%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR
Rate of compensation increase | 5.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 2.60%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 3.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR
Weighted-average assumptions used to determine net projected benefit cost: |  |  |
Discount rate | 4.10%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 5.20%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR
Expected long-term return on plan assets | 2.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 3.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 3.10%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR
Rate of compensation increase | 5.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 2.60%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR | 3.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease / simo_DefinedBenefitPlansAndOtherPostretirementBenefitPlansByRegionAxis = country_KR

X
- Definition The interest rate used to adjust for the time value of money for the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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X
- Definition Expected rate of compensation increases (for pay-related plans). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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X
- Definition The interest rate used to adjust for the time value of money. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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X
- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512013 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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