TAXATION - Aggregate Amount and Per Share Effect of the Preferential Tax Rate (Details) - CNY (¥) ¥ / shares in Units, ¥ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
TAXATION
Aggregate effect	¥ 107,547	¥ 81,300	¥ 75,864
Basic net income per share effect (in CNY per share)	¥ 1.79	¥ 1.40	¥ 1.31
Diluted net income per share effect (in CNY per share)	¥ 1.76	¥ 1.39	¥ 1.21

The amount of income taxes from which a reporting entity is exempt or for which it will receive a reduction in income taxes as a result of the income tax holiday granted by the taxing jurisdiction.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 11.C)
-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330215-122817

Name:	us-gaap_IncomeTaxHolidayAggregateDollarAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration