PARENT ONLY FINANCIAL INFORMATION (Condensed Balance Sheets) (Details)
¥ in Thousands
Dec. 31, 2017
CNY (¥)
shares
Dec. 31, 2017
$ / shares
Dec. 31, 2016
CNY (¥)
shares
Dec. 31, 2016
$ / shares
Dec. 31, 2015
CNY (¥)
Dec. 31, 2014
CNY (¥)
shares
Current assets:            
Time deposits ¥ 432,536   ¥ 416,724      
Prepaid expenses and other current assets 156,360   126,088      
Total current assets 1,492,518   1,450,858      
Total assets 2,282,645   2,085,012      
Current liabilities:            
Accrued expenses and other current liabilities 184,646   117,867      
Total current liabilities 624,131   479,749      
Other non-current liabilities 4,329   7,043      
Total liabilities 628,460   486,792      
Commitments and contingencies        
Shareholders' equity:            
Treasury shares (1,574,980 and 3,330,646 Class A ordinary shares as of December 31, 2016 and 2017, respectively, at cost) (255,103)   (93,761)      
Additional paid-in capital 1,094,872   995,216      
Accumulated other comprehensive income 54,122   58,204      
Retained earnings 759,893   638,173      
Total shareholders' equity 1,654,185   1,598,220   ¥ 1,338,614 ¥ 1,119,086
Total liabilities and shareholders' equity 2,282,645   2,085,012      
Class A [Member]            
Shareholders' equity:            
Ordinary shares ¥ 327   ¥ 302      
Ordinary shares            
Ordinary shares, par value | $ / shares   $ 0.001   $ 0.001    
Authorized | shares 860,000,000   860,000,000     860,000,000
Issued | shares 52,340,176   48,735,228     23,712,758
Outstanding | shares 49,009,530   47,160,248     23,712,758
Treasury shares | shares 3,330,646   1,574,980      
Class B [Member]            
Shareholders' equity:            
Ordinary shares ¥ 74   ¥ 86      
Ordinary shares            
Ordinary shares, par value | $ / shares   0.001   0.001    
Authorized | shares 40,000,000   40,000,000     40,000,000
Issued | shares 7,206,059   8,895,249     28,733,024
Outstanding | shares 7,206,059   8,895,249     28,733,024
Parent Company [Member]            
Current assets:            
Cash and cash equivalents ¥ 17,658   ¥ 43,770   ¥ 6,275 ¥ 7,931
Time deposits 65,342   242,795      
Prepaid expenses and other current assets 970   1,855      
Total current assets 83,970   288,420      
Investments and loans to subsidiaries 1,595,299   1,316,552      
Total assets 1,679,269   1,604,972      
Current liabilities:            
Accrued expenses and other current liabilities 24,454 [1]   3,409      
Total current liabilities 24,454   3,409      
Other non-current liabilities 630   3,343      
Total liabilities 25,084   6,752      
Commitments and contingencies        
Shareholders' equity:            
Treasury shares (1,574,980 and 3,330,646 Class A ordinary shares as of December 31, 2016 and 2017, respectively, at cost) (255,103)   (93,761)      
Additional paid-in capital 1,094,872   995,216      
Accumulated other comprehensive income 54,122   58,204      
Retained earnings 759,893   638,173      
Total shareholders' equity 1,654,185   1,598,220      
Total liabilities and shareholders' equity 1,679,269   1,604,972      
Parent Company [Member] | Class A [Member]            
Shareholders' equity:            
Ordinary shares ¥ 327   ¥ 302      
Ordinary shares            
Ordinary shares, par value | $ / shares   0.001   0.001    
Authorized | shares 860,000,000   860,000,000      
Issued | shares 52,340,176   48,735,228      
Outstanding | shares 49,009,530   4,716,024      
Treasury shares | shares 3,330,646   1,574,980      
Parent Company [Member] | Class B [Member]            
Shareholders' equity:            
Ordinary shares ¥ 74   ¥ 86      
Ordinary shares            
Ordinary shares, par value | $ / shares   $ 0.001   $ 0.001    
Authorized | shares 40,000,000   40,000,000      
Issued | shares 7,206,059   8,895,249      
Outstanding | shares 7,206,059   8,895,249      
[1] Mainly related to repurchase of treasury shares.