PRINCIPAL ACCOUNTING POLICIES - Share-Based Compensation(Details)	12 Months Ended
Dec. 31, 2017 $ / shares	Dec. 31, 2017 ¥ / shares	Dec. 31, 2016 $ / shares	Dec. 31, 2016 ¥ / shares	Dec. 31, 2015 $ / shares	Dec. 31, 2015 ¥ / shares
Share based compensation
Weighted average fair value per stock option on grant date (in dollars or CNY per share) \| (per share)	$ 15.62	¥ 101.63		¥ 66.87		¥ 68.90
Risk-free interest rate (as a percent)	1.61%	1.61%
Expected life (years)	4 years	4 years	4 years	4 years	4 years	4 years
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Volatility (as a percent)	32.00%	32.00%
Weighted average fair value of the common share on date of option grant	$ 56.68		$ 34.27		$ 31.40
Minimum
Share based compensation
Risk-free interest rate (as a percent)			1.00%	1.00%	1.14%	1.14%
Volatility (as a percent)			33.00%	33.00%	38.00%	38.00%
Maximum
Share based compensation
Risk-free interest rate (as a percent)			1.70%	1.70%	1.49%	1.49%
Volatility (as a percent)			34.00%	34.00%	42.00%	42.00%

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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No definition available.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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