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Financial Instruments - Summary of Maturity Analysis for Derivative Financial Instruments (Detail)
$ in Thousands, $ in Thousands
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Less than 1 month [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled $ (40,897) $ (1,379) $ (205,559)
1 to 3 months [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled (164,400) (5,547) (144,919)
3 months to 1 year [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled 227,325 7,669 655,299
Forward exchange contracts [member] | Less than 1 month [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Net settled (8,820) (298) 22,680
Gross settled 32,148 1,085 (111,528)
Forward exchange contracts [member] | 1 to 3 months [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Net settled     13,320
Gross settled 30,458 1,027 (3,687)
Forward exchange contracts [member] | 3 months to 1 year [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled 3,499 118  
Forward exchange contracts [member] | Inflows [member] | Less than 1 month [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled 3,711,302 125,213 5,134,196
Forward exchange contracts [member] | Inflows [member] | 1 to 3 months [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled 2,169,093 73,181 912,213
Forward exchange contracts [member] | Inflows [member] | 3 months to 1 year [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled 390,379 13,171  
Forward exchange contracts [member] | Outflows [member] | Less than 1 month [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled (3,679,154) (124,128) (5,245,724)
Forward exchange contracts [member] | Outflows [member] | 1 to 3 months [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled (2,138,635) (72,154) (915,900)
Forward exchange contracts [member] | Outflows [member] | 3 months to 1 year [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled (386,880) (13,154)  
Foreign currency option contracts [member] | Less than 1 month [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Net settled     (344)
Swap contracts [member] | Less than 1 month [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled (73,045) (2,464) (94,031)
Swap contracts [member] | 1 to 3 months [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled (194,858) (6,574) (141,232)
Swap contracts [member] | 3 months to 1 year [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled 223,826 7,551 655,299
Swap contracts [member] | Inflows [member] | Less than 1 month [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled 12,116,531 408,790 5,345,159
Swap contracts [member] | Inflows [member] | 1 to 3 months [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled 14,434,880 487,007 17,399,695
Swap contracts [member] | Inflows [member] | 3 months to 1 year [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled 36,676,224 1,237,389 43,537,500
Swap contracts [member] | Outflows [member] | Less than 1 month [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled (12,189,576) (411,254) (5,439,190)
Swap contracts [member] | Outflows [member] | 1 to 3 months [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled (14,629,738) (493,581) (17,540,927)
Swap contracts [member] | Outflows [member] | 3 months to 1 year [member]      
Disclosure of maturity analysis for derivative financial liabilities [line items]      
Gross settled $ (36,452,898) $ (1,229,838) $ (42,882,201)