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Stockholders' Equity - Summary of Common Stock (Parenthetical) (Detail) - shares
Dec. 31, 2017
Dec. 31, 2016
Disclosure of Classes of Share Capital [Line Items]    
Fixed portion of shares issued 13,068,000,000 13,068,000,000
Variable portion of shares issued 39,845,721,822 37,900,426,833
Series A [Member]    
Disclosure of Classes of Share Capital [Line Items]    
Percentage of capital stock 64.00%  
Series B [Member]    
Disclosure of Classes of Share Capital [Line Items]    
Percentage of capital stock 36.00%