INCOME TAX EXPENSE - ANALYSIS OF DIFFERENCES (Details) ¥ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019 CNY (¥)	Dec. 31, 2019 USD ($)	[1]	Dec. 31, 2018 CNY (¥)	Dec. 31, 2017 CNY (¥)
INCOME TAX EXPENSE [abstract]
Profit (loss) before tax	¥ 1,009,092	$ 144,948		¥ 1,068,800	¥ 1,347,132
Tax calculated at the statutory rate of 25%	252,273			267,200	336,783
Effect of expenses not deductible for tax purposes	2,921			19,647	663
Effect of income not subject to tax	(1,762)			(3,432)	(3,354)
Tax losses for which no deferred tax asset was recognized	11,249			9,098	5,561
Adjustments for current tax of prior periods	(2,410)			(2,335)	(3,886)
Utilization of previously unrecognized tax losses	(1,143)			(412)	(403)
Income tax expense	¥ 261,128	$ 37,509		¥ 289,766	¥ 335,364

[1] Translation of amounts from Renminbi ("RMB") into United States dollars ("US$") for the convenience of the reader has been made at US$1.00=RMB6.9618, the certified exchange rates for December 31, 2019 as published by the Federal Reserve Board of the United States. No representation is made that the RMB amounts could have been, or could be, converted into US$ at that rate on December 31, 2019 or on any other date.