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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Value-added-tax ("VAT") (Details) - CNY (¥)
¥ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Value-added-tax ("VAT")    
VAT recoverable ¥ 155,620 ¥ 129,994
VAT recoverable 1,445,521 888,483
Current assets    
Value-added-tax ("VAT")    
VAT recoverable 155,620 129,994
Noncurrent assets    
Value-added-tax ("VAT")    
VAT recoverable 1,445,521 888,483
Accrued expenses and other payables    
Value-added-tax ("VAT")    
VAT payables ¥ 6,838 ¥ 7,886