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Employee Defined Combination Plan - Additional Information (Detail)
$ in Millions, ¥ in Billions
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2017
CNY (¥)
Dec. 31, 2016
CNY (¥)
Dec. 31, 2015
CNY (¥)
Defined Contribution Plan [Abstract]        
Amounts incurred for employee defined contribution plan $ 402.0 ¥ 2.6 ¥ 2.3 ¥ 2.2