Profit Before Income Tax - Summary of Finance Costs (Detail) $ in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2017 TWD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 TWD ($)	Dec. 31, 2015 TWD ($)
Analysis of income and expense [abstract]
Total interest expense for financial liabilities measured at amortized cost	$ 2,016,298	$ 68,026	$ 2,510,197	$ 2,514,208
Less: Amounts included in the cost of qualifying assets
Inventories related to real estate business	(190,137)	(6,415)	(238,469)	(197,287)
Property, plant and equipment	(51,262)	(1,729)	(54,191)	(48,135)
Investment property	(13)
Interest expense	1,774,886	59,882	2,217,537	2,268,786
Other finance costs	24,608	830	43,538	43,357
Finance costs	$ 1,799,494	$ 60,712	$ 2,261,075	$ 2,312,143

Amounts included in the cost of qualifying assets.

No definition available.

Name:	asx_AmountsIncludedInTheCostOfQualifyingAssetsAbstract
Namespace Prefix:	asx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Borrowing costs capitalised investment property.

+ References

No definition available.

+ Details

Name:	asx_BorrowingCostsCapitalisedInvestmentProperty
Namespace Prefix:	asx_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Interest expense excluding loss on derivatives.

+ References

No definition available.

+ Details

Name:	asx_InterestExpenseExcludingLossOnDerivatives
Namespace Prefix:	asx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Interest expense on financial liabilities measured at amortized cost.

+ References

No definition available.

+ Details

Name:	asx_InterestExpenseOnFinancialLiabilitiesMeasuredAtAmortizedCost
Namespace Prefix:	asx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Inventories related to real estate business costs capitalised.

+ References

No definition available.

+ Details

Name:	asx_InventoriesRelatedToRealEstateBusinessCostsCapitalised
Namespace Prefix:	asx_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

The amount of interest and other costs that an entity incurs in connection with the borrowing of funds that are directly attributable to the acquisition, construction or production of a qualifying asset and which form part of the cost of that asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 23
-IssueDate 2017-01-01
-Paragraph 26
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=23&date=2017-03-01&anchor=para_26_a&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_BorrowingCostsCapitalised
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of costs associated with financing activities of the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 82
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_82_b&doctype=Standard
-URIDate 2017-03-09

+ Details

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

- Definition

The amount of finance costs that the entity does not separately disclose in the same statement or note. [Refer: Finance costs]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

+ Details

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