PRINCIPAL ACCOUNTING POLICIES (Cash, cash equivalents and restricted cash) (Details)	Dec. 31, 2018 CNY (¥)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 CNY (¥)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 CNY (¥)	Dec. 31, 2015 CNY (¥)
Cash and cash equivalent	¥ 3,104,916,803	$ 451,591,419	¥ 1,928,302,849
Restricted cash	377,110,770	54,848,487	833,071,955
Total	¥ 3,482,027,573	$ 506,439,906	¥ 2,761,374,804	$ 401,625,308	¥ 2,820,202,414	¥ 2,948,315,006

Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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