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Reconciliation of Income Tax Expense on Pretax Income at Statutory Rate and Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation Of Income Taxes [Line Items]
Cayman statutory rate	$ 0simo_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtUnifiedStatutoryIncomeTaxRate	$ 0simo_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtUnifiedStatutoryIncomeTaxRate	$ 0simo_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtUnifiedStatutoryIncomeTaxRate
Tax on pretax income at statutory rate	15,727us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	6,788us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	8,142us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
Tax-exempt income	(2,573)us-gaap_IncomeTaxReconciliationTaxExemptIncome	(4,325)us-gaap_IncomeTaxReconciliationTaxExemptIncome	(5,149)us-gaap_IncomeTaxReconciliationTaxExemptIncome
Permanent differences	(396)simo_IncomeTaxReconciliationPermanentDifferences	1,730simo_IncomeTaxReconciliationPermanentDifferences	861simo_IncomeTaxReconciliationPermanentDifferences
Temporary differences	(344)simo_TemporaryDifferences	1,732simo_TemporaryDifferences	(2,290)simo_TemporaryDifferences
Alternative minimum tax	1,170simo_IncomeTaxReconciliationAlternativeMinimumTax	2,203simo_IncomeTaxReconciliationAlternativeMinimumTax	4,340simo_IncomeTaxReconciliationAlternativeMinimumTax
Income tax (10%) on undistributed earnings	2,491simo_IncomeTaxReconciliationInUndistributedEarnings	3,396simo_IncomeTaxReconciliationInUndistributedEarnings	2,631simo_IncomeTaxReconciliationInUndistributedEarnings
Net changes in income tax credit	(899)us-gaap_IncomeTaxReconciliationTaxCreditsInvestment	708us-gaap_IncomeTaxReconciliationTaxCreditsInvestment	5,518us-gaap_IncomeTaxReconciliationTaxCreditsInvestment
Net changes in valuation allowance of deferred income tax assets	733us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	(2,364)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	(3,879)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Net operating loss carryforwards	(1,298)simo_IncomeTaxReconciliationUtilizationOfOperatingLossCarryforwards	(189)simo_IncomeTaxReconciliationUtilizationOfOperatingLossCarryforwards	604simo_IncomeTaxReconciliationUtilizationOfOperatingLossCarryforwards
Liabilities related to unrealized tax benefits	91us-gaap_IncomeTaxReconciliationOtherAdjustments	(39)us-gaap_IncomeTaxReconciliationOtherAdjustments	832us-gaap_IncomeTaxReconciliationOtherAdjustments
Adjustment of prior years' taxes and others	1,399us-gaap_IncomeTaxReconciliationPriorYearIncomeTaxes	132us-gaap_IncomeTaxReconciliationPriorYearIncomeTaxes	(4,494)us-gaap_IncomeTaxReconciliationPriorYearIncomeTaxes
Income tax expense	$ 16,101us-gaap_IncomeTaxExpenseBenefit	$ 9,772us-gaap_IncomeTaxExpenseBenefit	$ 7,116us-gaap_IncomeTaxExpenseBenefit

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Income Tax Reconciliation Income Tax Expense (Benefit) at Unified Statutory Income Tax Rate

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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