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PRINCIPAL ACCOUNTING POLICIES - Goodwill and other intangible assets to sales and marketing (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2017
CNY (¥)
Dec. 31, 2016
CNY (¥)
Dec. 31, 2015
CNY (¥)
Dec. 31, 2017
CNY (¥)
Intangible assets          
Impairment of goodwill   ¥ 0 ¥ 0 ¥ 0  
Impairment on other intangible assets   0 0 0  
Accrued liability for customer reward program          
Accrued liability for customer reward program $ 93,697,131   658,170,680   ¥ 609,621,643
Amount of expenses for group's customers reward program   100,000,000 202,000,000 399,000,000  
Allowance for doubtful accounts activity          
Balance at beginning of year   58,841,826 38,237,658 14,707,184  
Provision for doubtful accounts $ 15,053,760 97,944,278 32,339,617 32,080,786  
Write-offs   (28,064,747) (11,735,449) (8,550,312)  
Balance at end of period   128,721,357 58,841,826 38,237,658  
Sales and marketing          
Advertising expenses   ¥ 5,100,000,000 ¥ 2,800,000,000 ¥ 1,800,000,000  
Minimum          
Intangible assets          
Intangible asset useful life 3 years 3 years      
Maximum          
Intangible assets          
Intangible asset useful life 10 years 10 years