Goodwill - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Impairment loss of goodwill	$ 0	$ 0
Accumulated impairment losses, net		102,000,000	$ 102,000,000
Microcontrollers and Digital ICs Group (MDG) [Member]
Goodwill [Line Items]
Accumulated impairment losses, net		96,000,000	96,000,000
Others [Member]
Goodwill [Line Items]
Accumulated impairment losses, net		$ 6,000,000	$ 6,000,000

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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