PRINCIPAL ACCOUNTING POLICIES - CHANGES IN ACCOUNTING POLICIES - Impact on the financial statements (Details) $ in Thousands	Dec. 31, 2018 CNY (¥)	Dec. 31, 2018 USD ($)	[1]	Jan. 01, 2018 CNY (¥)	Dec. 31, 2017 CNY (¥)	Dec. 31, 2016 CNY (¥)	Dec. 31, 2015 CNY (¥)
Non-current assets
Financial assets at fair value through other comprehensive income	¥ 321,246,000	$ 46,723		¥ 296,414,000	¥ 0
Available-for-sale investments	0	0		0	296,414,000
Deferred tax assets	197,295,000	28,695		53,563,000	37,005,000
Total non-current assets	29,045,560,000	4,224,502		349,977,000	27,936,352,000
Current assets [abstract]
Trade receivables	3,861,617,000	561,649		4,081,506,000	4,142,210,000
Prepayments and other receivables	348,907,000	50,747		308,724,000	314,251,000
Total current assets	6,356,677,000	924,541		4,390,230,000	6,057,886,000
Non-current liabilities [abstract]
Deferred income	99,765,000	14,510		105,754,000	105,791,000
Current liabilities [abstract]
Contract liabilities	203,631,000	29,617		135,499,000	0
Accruals and other payables	2,076,798,000	302,058		1,327,769,000	1,463,231,000
Total current liabilities	6,422,245,000	934,077		1,463,268,000	5,164,975,000
Equity
Other reserves	3,188,161,000	463,699		3,104,549,000	3,109,516,000
Retained earnings	7,017,944,000	1,020,718		6,884,180,000	6,928,886,000
Total equity	¥ 28,816,329,000	$ 4,191,162		9,988,729,000	28,657,081,000	¥ 28,030,055,000	¥ 27,444,262,000
Previously stated [member]
Non-current assets
Financial assets at fair value through other comprehensive income					0
Available-for-sale investments					296,414,000
Deferred tax assets					37,005,000
Total non-current assets					333,419,000
Current assets [abstract]
Trade receivables					4,142,210,000
Prepayments and other receivables					314,251,000
Total current assets					4,456,461,000
Non-current liabilities [abstract]
Deferred income					105,791,000
Current liabilities [abstract]
Contract liabilities					0
Accruals and other payables					1,463,231,000
Total current liabilities					1,463,231,000
Equity
Other reserves					3,109,516,000
Retained earnings					6,928,886,000
Total equity					¥ 10,038,402,000
Adjustment from Adoption of IFRS 9
Non-current assets
Financial assets at fair value through other comprehensive income				296,414,000
Available-for-sale investments				(296,414,000)
Deferred tax assets				16,558,000
Total non-current assets				16,558,000
Current assets [abstract]
Trade receivables				(60,704,000)
Prepayments and other receivables				(5,527,000)
Total current assets				(66,231,000)
Non-current liabilities [abstract]
Deferred income				0
Current liabilities [abstract]
Contract liabilities				0
Accruals and other payables				0
Total current liabilities				0
Equity
Other reserves				(4,967,000)
Retained earnings				(44,706,000)
Total equity				(49,673,000)
Adjustment from Adoption of IFRS 15
Non-current assets
Financial assets at fair value through other comprehensive income				0
Available-for-sale investments				0
Deferred tax assets				0
Total non-current assets				0
Current assets [abstract]
Trade receivables				0
Prepayments and other receivables				0
Total current assets				0
Non-current liabilities [abstract]
Deferred income				(37,000)
Current liabilities [abstract]
Contract liabilities				135,499,000
Accruals and other payables				(135,462,000)
Total current liabilities				37,000
Equity
Other reserves				0
Retained earnings				0
Total equity				¥ 0

[1] Translation of amounts from Renminbi (" RMB ") into United States dollars ("US$") for the convenience of the reader has been made at US$1.00=RMB6. 8755 , the certified exchange rates for December 31 , 201 8 as published by the Federal Reserve Board of the United States . No representation is made that the RMB amounts could have been, or could be, converted into US$ at that rate on December 31 , 201 8 or on any other date .