Segment Information - Financial Segment Information for Operating Segments (Detail) $ in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2019 TWD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 TWD ($)	Dec. 31, 2017 TWD ($)
Revenue
Revenue	$ 20,337,881	$ 679,969	$ 18,480,027	$ 17,940,855
Operating profit (loss)	2,457,119	82,150	2,099,721	2,239,881
Interest expense	(171,075)		(152,416)	(190,425)
Share of profit (loss) of associates	(154,926)	$ (5,179)	(300,101)	(179,491)
Purchase of property, plant and equipment	4,896,656		4,945,570
Continuing operations [member]
Revenue
Revenue	20,337,881		18,480,027	17,940,855
Operating profit (loss)	2,457,119		2,099,721	2,239,881
Depreciation expenses	(3,731,914)		(3,376,579)	(2,899,278)
Interest income	64,368		49,971	53,123
Interest expense	(171,075)		(152,416)	(190,425)
Share of profit (loss) of associates	(154,926)		(300,101)	(179,491)
Purchase of property, plant and equipment	4,896,656		4,945,570	4,702,691
Continuing operations [member] \| External [member]
Revenue
Revenue	20,337,881		18,480,027	17,940,855
Continuing operations [member] \| Operating segments [member] \| Testing [member]
Revenue
Revenue	4,257,800		4,790,097	4,838,246
Operating profit (loss)	709,142		1,306,742	1,448,939
Depreciation expenses	(802,740)		(769,660)	(673,393)
Purchase of property, plant and equipment	764,105		1,563,919	836,894
Continuing operations [member] \| Operating segments [member] \| Testing [member] \| External [member]
Revenue
Revenue	4,257,800		4,790,097	4,838,246
Continuing operations [member] \| Operating segments [member] \| Assembly [member]
Revenue
Revenue	5,148,877		4,679,676	5,259,281
Operating profit (loss)	(227,096)		(207,700)	(55,198)
Depreciation expenses	(521,311)		(578,205)	(597,500)
Purchase of property, plant and equipment	548,063		321,976	655,879
Continuing operations [member] \| Operating segments [member] \| Assembly [member] \| External [member]
Revenue
Revenue	5,148,877		4,679,676	5,259,281
Continuing operations [member] \| Operating segments [member] \| LCDD [member]
Revenue
Revenue	6,922,205		5,694,720	4,790,116
Operating profit (loss)	1,740,720		1,226,755	1,285,495
Depreciation expenses	(1,796,951)		(1,400,784)	(1,048,587)
Purchase of property, plant and equipment	3,077,806		2,732,141	2,615,153
Continuing operations [member] \| Operating segments [member] \| LCDD [member] \| External [member]
Revenue
Revenue	6,922,205		5,694,720	4,789,869
Continuing operations [member] \| Operating segments [member] \| LCDD [member] \| Intersegment [member]
Revenue
Revenue				247
Continuing operations [member] \| Operating segments [member] \| Bumping [member]
Revenue
Revenue	4,008,999		3,315,534	3,053,459
Operating profit (loss)	232,404		(202,497)	(336,123)
Depreciation expenses	(604,553)		(627,412)	(579,605)
Purchase of property, plant and equipment	506,635		327,251	594,765
Continuing operations [member] \| Operating segments [member] \| Bumping [member] \| External [member]
Revenue
Revenue	4,008,999		3,315,534	3,053,459
Continuing operations [member] \| Operating segments [member] \| Others [member]
Revenue
Revenue	32,808		35,738	35,808
Operating profit (loss)	1,931		(23,433)	(100,545)
Depreciation expenses	(6,359)		(518)	(503)
Interest income	64,368		49,971	53,123
Interest expense	(171,075)		(152,416)	(190,425)
Share of profit (loss) of associates	(370,351)		(668,377)	1,347,851
Purchase of property, plant and equipment	47		283
Continuing operations [member] \| Operating segments [member] \| Others [member] \| Intersegment [member]
Revenue
Revenue	32,808		35,738	35,808
Continuing operations [member] \| Elimination [member]
Revenue
Revenue	(32,808)		(35,738)	(36,055)
Operating profit (loss)	18		(146)	(2,687)
Depreciation expenses				310
Share of profit (loss) of associates	215,425		368,276	(1,527,342)
Continuing operations [member] \| Elimination [member] \| Intersegment [member]
Revenue
Revenue	$ (32,808)		$ (35,738)	(36,055)
Discontinued operations: Unimos Shanghai [member]
Revenue
Revenue				227,095
Operating profit (loss)				(25,394)
Interest income				464
Interest expense				(2,414)
Discontinued operations: Unimos Shanghai [member] \| External [member]
Revenue
Revenue				$ 227,095

The amount of additions to property, plant and equipment other than those acquired through business combinations. [Refer: Business combinations [member]; Property, plant and equipment]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 16
-IssueDate 2019-01-01
-Paragraph 73
-Subparagraph e
-Clause i
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=16&code=ifrs-tx-2019-en-r&anchor=para_73_e_i&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_AdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipment
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of depreciation expense. Depreciation is the systematic allocation of depreciable amounts of tangible assets over their useful lives.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_DepreciationExpense
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of income associated with interest and other financing activities of the entity.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 85
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_85&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_FinanceIncome
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of expense arising from interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 23
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_23_d&doctype=Standard
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 28
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_28_e&doctype=Standard
-URIDate 2019-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2019-01-01
-Paragraph B13
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=12&code=ifrs-tx-2019-en-r&anchor=para_B13_f&doctype=Appendix&subtype=B
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_InterestExpense
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The profit (loss) from operating activities of the entity. [Refer: Profit (loss)]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 32
-IssueDate 2019-01-01
-Paragraph IE33
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=32&code=ifrs-tx-2019-en-r&anchor=para_IE33&doctype=Illustrative%20Examples
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 85
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_85&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_ProfitLossFromOperatingActivities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Expiry date 2020-01-01: The income arising in the course of an entity's ordinary activities. Income is increases in economic benefits during the accounting period in the form of inflows or enhancements of assets or decreases of liabilities that result in an increase in equity, other than those relating to contributions from equity participants. Effective 2020-01-01: The income arising in the course of an entity's ordinary activities. Income is increases in assets, or decreases in liabilities, that result in increases in equity, other than those relating to contributions from holders of equity claims.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 28
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_28_a&doctype=Standard
-URIDate 2019-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 23
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_23_a&doctype=Standard
-URIDate 2019-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 32
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_32&doctype=Standard
-URIDate 2019-03-27

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 33
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_33_a&doctype=Standard
-URIDate 2019-03-27

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 8
-IssueDate 2019-01-01
-Paragraph 34
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=8&code=ifrs-tx-2019-en-r&anchor=para_34&doctype=Standard
-URIDate 2019-03-27

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 82
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_82_a&doctype=Standard
-URIDate 2019-03-27

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 103
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_103&doctype=Standard
-URIDate 2019-03-27

Reference 8: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 102
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_102&doctype=Standard
-URIDate 2019-03-27

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 5
-IssueDate 2019-01-01
-Paragraph 33
-Subparagraph b
-Clause i
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=5&code=ifrs-tx-2019-en-r&anchor=para_33_b_i&doctype=Standard
-URIDate 2019-03-27

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2019-01-01
-Paragraph B12
-Subparagraph b
-Clause v
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=12&code=ifrs-tx-2019-en-r&anchor=para_B12_b_v&doctype=Appendix&subtype=B
-URIDate 2019-03-27

Reference 11: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 12
-IssueDate 2019-01-01
-Paragraph B10
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=12&code=ifrs-tx-2019-en-r&anchor=para_B10_b&doctype=Appendix&subtype=B
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_Revenue
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	ifrs-full_RevenueAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The entity's share of the profit (loss) of associates accounted for using the equity method. [Refer: Associates [member]; Investments accounted for using equity method; Profit (loss)]

+ References

+ Details

Name:	ifrs-full_ShareOfProfitLossOfAssociatesAccountedForUsingEquityMethod
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	ifrs-full_ContinuingAndDiscontinuedOperationsAxis=imos_ContinuingOperationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfCustomersAxis=imos_ExternalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SegmentConsolidationItemsAxis=ifrs-full_OperatingSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SegmentsAxis=imos_TestingSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SegmentsAxis=imos_AssemblySegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SegmentsAxis=imos_LCDDSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfCustomersAxis=imos_IntersegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SegmentsAxis=imos_BumpingSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SegmentsAxis=ifrs-full_AllOtherSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SegmentConsolidationItemsAxis=ifrs-full_EliminationOfIntersegmentAmountsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ContinuingAndDiscontinuedOperationsAxis=ifrs-full_DiscontinuedOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: