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Expenses by nature - Summary of Expenses by nature (Detail) - TWD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Analysis of income and expense [abstract]      
Raw materials and supplies used $ 4,708,493 $ 3,575,283 $ 3,079,909
Employee benefit expenses 6,010,227 6,075,773 5,606,833
Depreciation expenses 4,175,519 3,731,914 3,376,579
Others 4,686,218 4,590,720 4,464,499
Total operating costs and expenses $ 19,580,457 $ 17,973,690 $ 16,527,820