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Retirement Benefit Plans - Summary of Movements in Net Defined Benefit Liability (Asset) (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2016
TWD ($)
Disclosure of net defined benefit liability (asset) [line items]        
Beginning balance $ 116,604 $ 3,569,265 $ 3,972,517 $ 3,999,947
Current service cost 7,322 224,126 278,412 329,838
Past service cost and gain on settlements     (68,979)  
Net interest expense (income) 1,832 56,070 53,663 58,031
Recognized in profit or loss 9,154 280,196 263,096 387,869
Return on plan assets (excluding amounts included in net interest) (542) (16,589) 52,124 54,549
Actuarial loss (gain) arising from changes in financial assumptions (282) (8,643) 56,860 156,193
Actuarial loss (gain) arising from experience adjustments 9,882 302,499 (315,090) 200,723
Actuarial loss arising from changes in demographic assumptions 268 8,190 762 5,716
Actuarial gain arising from changes in other assumptions 742 22,723    
Recognized in other comprehensive income 10,068 308,180 (205,344) 417,181
Contributions from the employer (11,899) (364,237) (484,790) (807,232)
Benefits paid from the Group (9,669) (295,953) (96,575) (36,033)
Business combinations 42,871 1,312,281    
Exchange differences on foreign plans (154) (4,716) 120,361 10,785
Ending balance 156,975 4,805,016 3,569,265 3,972,517
Defined benefit obligation [member]        
Disclosure of net defined benefit liability (asset) [line items]        
Beginning balance 258,433 7,910,638 8,389,884 7,973,676
Current service cost 7,322 224,126 278,412 329,838
Past service cost and gain on settlements     (68,979)  
Net interest expense (income) 5,841 178,779 157,404 167,111
Recognized in profit or loss 13,163 402,905 366,837 496,949
Actuarial loss (gain) arising from changes in financial assumptions (282) (8,643) 56,860 156,193
Actuarial loss (gain) arising from experience adjustments 9,882 302,499 (315,090) 200,723
Actuarial loss arising from changes in demographic assumptions 268 8,190 762 5,716
Actuarial gain arising from changes in other assumptions 742 22,723    
Recognized in other comprehensive income 10,610 324,769 (257,468) 362,632
Benefits paid from the pension fund (17,706) (541,989) (690,830) (308,471)
Benefits paid from the Group (9,669) (295,953) (96,575) (36,033)
Business combinations 82,418 2,522,805    
Liabilities assumed in a business combination       535
Exchange differences on foreign plans (851) (26,036) 198,790 (99,404)
Ending balance 336,398 10,297,139 7,910,638 8,389,884
Fair value of plan assets [member]        
Disclosure of net defined benefit liability (asset) [line items]        
Beginning balance (141,829) (4,341,373) (4,417,367) (3,973,729)
Net interest expense (income) (4,009) (122,709) (103,741) (109,080)
Recognized in profit or loss (4,009) (122,709) (103,741) (109,080)
Return on plan assets (excluding amounts included in net interest) (542) (16,589) 52,124 54,549
Recognized in other comprehensive income (542) (16,589) 52,124 54,549
Contributions from the employer (11,899) (364,237) (484,790) (807,232)
Benefits paid from the pension fund 17,706 541,989 690,830 308,471
Business combinations (39,547) (1,210,524)    
Liabilities assumed in a business combination       (535)
Exchange differences on foreign plans 697 21,320 (78,429) 110,189
Ending balance $ (179,423) $ (5,492,123) $ (4,341,373) $ (4,417,367)