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Changes in Benefits Obligation And Plan Assets And Reconciliation of Funded Status (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in benefit obligation
Projected benefit obligation at beginning of year	$ 2,098us-gaap_DefinedBenefitPlanBenefitObligation	$ 1,897us-gaap_DefinedBenefitPlanBenefitObligation	$ 1,458us-gaap_DefinedBenefitPlanBenefitObligation
Service cost	437us-gaap_DefinedBenefitPlanServiceCost	440us-gaap_DefinedBenefitPlanServiceCost	608us-gaap_DefinedBenefitPlanServiceCost
Interest cost	58us-gaap_DefinedBenefitPlanInterestCost	58us-gaap_DefinedBenefitPlanInterestCost	77us-gaap_DefinedBenefitPlanInterestCost
Actuarial loss (gain)	814us-gaap_DefinedBenefitPlanActuarialGainLoss	(202)us-gaap_DefinedBenefitPlanActuarialGainLoss	(187)us-gaap_DefinedBenefitPlanActuarialGainLoss
Benefits paid	(87)us-gaap_DefinedBenefitPlanBenefitsPaid	(95)us-gaap_DefinedBenefitPlanBenefitsPaid	(59)us-gaap_DefinedBenefitPlanBenefitsPaid
Projected benefit obligation at end of year	3,320us-gaap_DefinedBenefitPlanBenefitObligation	2,098us-gaap_DefinedBenefitPlanBenefitObligation	1,897us-gaap_DefinedBenefitPlanBenefitObligation
Change in plan assets
Fair value of plan assets at beginning of year	2,319us-gaap_DefinedBenefitPlanFairValueOfPlanAssets	1,925us-gaap_DefinedBenefitPlanFairValueOfPlanAssets	1,376us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Actual return on plan assets	31us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets	43us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets	43us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
Employer contributions	282us-gaap_DefinedBenefitPlanContributionsByEmployer	433us-gaap_DefinedBenefitPlanContributionsByEmployer	546us-gaap_DefinedBenefitPlanContributionsByEmployer
Benefits paid	(76)simo_DefinedBenefitPlanChangeInFairValueOfPlanAssetsBenefitsPaid	(82)simo_DefinedBenefitPlanChangeInFairValueOfPlanAssetsBenefitsPaid	(40)simo_DefinedBenefitPlanChangeInFairValueOfPlanAssetsBenefitsPaid
Fair value of plan assets at end of year	2,556us-gaap_DefinedBenefitPlanFairValueOfPlanAssets	2,319us-gaap_DefinedBenefitPlanFairValueOfPlanAssets	1,925us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Funded status recognized as an other asset (liabilities)	$ (764)us-gaap_DefinedBenefitPlanFundedStatusOfPlan	$ 221us-gaap_DefinedBenefitPlanFundedStatusOfPlan	$ 28us-gaap_DefinedBenefitPlanFundedStatusOfPlan

X

- Definition

Defined Benefit Plan Change In Fair Value Of Plan Assets Benefits Paid

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- Definition

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan.

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1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

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The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

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The increase in the fair value of plan assets from contributions made by the employer.

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Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

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The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

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The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

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The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

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