Depreciation and amortization of property & equipment.

No definition available.

Name:	simo_DepreciationAndAmortizationOfPropertyAndEquipment
Namespace Prefix:	simo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Held To Maturity Investments Contractual Maturity

+ References

No definition available.

+ Details

Name:	simo_HeldToMaturityInvestmentsContractualMaturity
Namespace Prefix:	simo_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Customers Accounting For Ten Percent Or More Of Consolidated Revenue

+ References

No definition available.

+ Details

Name:	simo_NumberOfCustomersAccountingForTenPercentOrMoreOfConsolidatedRevenue
Namespace Prefix:	simo_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage Of Income Tax Examination Minimum Likelihood of Tax Benefits Being Realized Upon Ultimate Settlement

+ References

No definition available.

+ Details

Name:	simo_PercentageOfIncomeTaxExaminationMinimumLikelihoodOfTaxBenefitsBeingRealizedUponUltimateSettlement
Namespace Prefix:	simo_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Short Term Investments, Maturity Period

+ References

No definition available.

+ Details

Name:	simo_ShortTermInvestmentsMaturityPeriod
Namespace Prefix:	simo_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Significant Accounting Policies [Line Items]

+ References

No definition available.

+ Details

Name:	simo_SignificantAccountingPoliciesLineItems
Namespace Prefix:	simo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

+ Details

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

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13537-108611

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of consideration received or receivable for the disposal of assets and liabilities, including discontinued operation.

+ References

No definition available.

+ Details

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_EscrowDeposit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) on sale or disposal of equity in securities of subsidiaries or equity method investee.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(b)(9)(a))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(g))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(b)(7)(c))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(3)(a))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

+ Details

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

- Definition

Amount of current assets that are pledged or subject to withdrawal restrictions, classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_OtherRestrictedAssetsCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of reorganization value related to estimated net realizable value of asset dispositions. Reorganization value is the amount attributed to the reconstituted entity, as well as the expected net realizable value of those assets that will be disposed before reconstitution occurs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 05
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=49179835&loc=d3e54813-112758

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56239-112766

+ Details

Name:	us-gaap_ReorganizationValueNetRealizableValueOfAssetDispositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of subsidiary's or equity investee's stock owned by parent company after stock transaction.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockPercentageOfOwnershipAfterTransaction
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=simo_DialogueSemiconductorPlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=simo_FCIIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_SalesRevenueNetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=simo_TopTenCustomersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=simo_TwoCustomersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis=simo_EmployeeStockOptionAndRestrictedStockUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=simo_AmericanDepositaryShareMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_DevelopedTechnologyRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_ComputerSoftwareIntangibleAssetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_MachineryAndEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_FurnitureAndFixturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LeaseholdImprovementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Summary of Significant Accounting Policies - Additional Information (Detail) shares in Thousands, $ in Thousands		12 Months Ended
	May 31, 2019 USD ($)	Dec. 31, 2020 USD ($) Customer shares	Dec. 31, 2019 USD ($) Customer shares	Dec. 31, 2018 USD ($) Customer shares
Significant Accounting Policies [Line Items]
Subsidiary ownership percentage		100.00%
Number of customers accounted for 10% or more sales \| Customer		1	2	2
Depreciation and amortization of property & equipment		$ 13,562	$ 12,447	$ 11,832
Evaluation of tax benefits realized upon settlement		50.00%
Gain loss on disposal of subsidary		$ (293)	12,409
Dialogue Semiconductor PLC [Member] \| FCI
Significant Accounting Policies [Line Items]
Disposal group discontinued operation consideration receivable	$ 54,000
Gain loss on disposal of subsidary	12,409
Disposal group discontinued operations net asset value	$ 39,367
Restricted assets net		$ 5,400	5,400
Escrow Deposit			$ 5,400
Minimum
Significant Accounting Policies [Line Items]
Short term investment maturity period		3 months
Maximum
Significant Accounting Policies [Line Items]
Short term investment maturity period		1 year
Held-to-maturity investment maturity period		1 year
Net sales \| Customers accounted for 10% or more \| Top Ten Customers [Member]
Significant Accounting Policies [Line Items]
Major customers percentage of net sales		71.00%	74.00%	69.00%
Net sales \| Customers accounted for 10% or more \| Two Customers [Member]
Significant Accounting Policies [Line Items]
Major customers percentage of net sales		24.00%	31.00%	34.00%
Employee Stock Option And Restricted Stock Units [Member]
Significant Accounting Policies [Line Items]
The effect of dilutive securities of employee stock options and restricted stock units \| shares		489	475	389
Employee Stock Option And Restricted Stock Units [Member] \| American Depositary Shares
Significant Accounting Policies [Line Items]
The effect of dilutive securities of employee stock options and restricted stock units \| shares		123	119	97
Development technology \| Minimum
Significant Accounting Policies [Line Items]
Intangible asset amortized over estimated useful lives		3 years 6 months
Development technology \| Maximum
Significant Accounting Policies [Line Items]
Intangible asset amortized over estimated useful lives		5 years 6 months
Software \| Minimum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		1 year
Software \| Maximum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		5 years
Buildings \| Minimum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		25 years
Buildings \| Maximum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		50 years
Machinery and Equipment \| Minimum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		3 years
Machinery and Equipment \| Maximum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		6 years
Furniture and Fixtures \| Minimum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		3 years
Furniture and Fixtures \| Maximum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		8 years
Leasehold And Building Improvement \| Minimum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		2 years
Leasehold And Building Improvement \| Maximum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful life		6 years