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Cash and Cash Equivalents - Annual Yield Rates of Bank Deposits, Commercial Paper, Negotiable Certificate of Deposit and Time Deposits (Detail)
Dec. 31, 2017
Dec. 31, 2016
Bank deposits [Member] | Bottom of range [Member]    
Disclosure of cash and cash equivalents [Line Items]    
Yield rate 0.00% 0.00%
Bank deposits [Member] | Top of range [Member]    
Disclosure of cash and cash equivalents [Line Items]    
Yield rate 0.70% 0.42%
Commercial paper [Member] | Bottom of range [Member]    
Disclosure of cash and cash equivalents [Line Items]    
Yield rate 0.32% 0.32%
Commercial paper [Member] | Top of range [Member]    
Disclosure of cash and cash equivalents [Line Items]    
Yield rate 0.40% 0.42%
Negotiable certificate of deposit [Member] | Bottom of range [Member]    
Disclosure of cash and cash equivalents [Line Items]    
Yield rate 0.40% 0.35%
Negotiable certificate of deposit [Member] | Top of range [Member]    
Disclosure of cash and cash equivalents [Line Items]    
Yield rate 0.50% 0.50%
Time deposits [Member] | Bottom of range [Member]    
Disclosure of cash and cash equivalents [Line Items]    
Yield rate 0.52% 0.40%
Time deposits [Member] | Top of range [Member]    
Disclosure of cash and cash equivalents [Line Items]    
Yield rate 4.40% 3.30%