Income Tax - Reconciliation of Income before Income Tax and Income Tax Expense Recognized in Profit or Loss (Detail) $ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2017 TWD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 TWD ($)	Dec. 31, 2015 TWD ($)
Reconciliation of accounting profit multiplied by applicable tax rates [abstract]
Income before tax	$ 396,161.9		$ 385,921.7	$ 350,477.6
Income tax expense at the statutory rate	69,613.5		66,938.7	60,674.4
Tax effect of adjusting items:
Deductible items in determining taxable income	(1,415.9)		(44.9)	(6,340.4)
Tax-exempt income	(16,901.1)		(19,595.0)	(22,144.3)
Additional income tax under the Alternative Minimum Tax Act				6,041.6
Additional income tax on unappropriated earnings	28,183.5		30,046.8	27,543.6
Effect of tax rate changes on deferred income tax	561.8
The origination and reversal of temporary differences	(4,336.1)		(1,775.0)	(1,542.8)
Income tax credits	(5,628.6)		(4,940.2)	(4,243.6)
Remeasurement of operating loss carryforward			(0.4)	69.6
Tax effect of adjusting items	70,077.1		70,630.0	60,058.1
Income tax adjustments on prior years	(19,107.0)		(16,628.1)	(12,661.2)
Other income tax adjustments	152.8		122.5	247.8
Income tax expense recognized in profit or loss	$ 51,122.9	$ 1,724.8	$ 54,124.4	$ 47,644.7

The amount of profit (loss) for a period before deducting tax expense. [Refer: Profit (loss)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2017-01-01
-Paragraph 81
-Subparagraph c
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2017-03-01&anchor=para_81_c_ii&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2017-01-01
-Paragraph 81
-Subparagraph c
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2017-03-01&anchor=para_81_c_i&doctype=Standard
-URIDate 2017-03-09