Summary of Principal Accounting Policies (Details 2) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Financial statement balances included in the consolidated financial statements        
Cash and cash equivalents $ 630,616,635 $ 413,319,174 $ 210,841,368 $ 392,005,353
Accounts receivable, net of allowance for doubtful accounts 415,150,008 357,442,102    
Other current assets 39,339,526 44,234,499    
Amounts due from related parties 6,094,260 1,263,416    
Total current assets 1,376,028,730 1,029,163,781    
TOTAL ASSETS 1,776,923,241 1,355,520,098    
Accounts payable 8,260,681 11,264,939    
Accrued payroll and welfare expenses 116,577,317 102,632,157    
Income tax payable 117,593,159 98,685,965    
Other tax payable 49,390,175 40,000,505    
Amounts due to related parties 7,356,186 5,535,512    
Advance from customers and deferred revenue 19,013,041 24,617,144    
Liability for exclusive rights, current   8,967,972    
Other current liabilities 85,836,572 62,466,610    
Total current liabilities 455,144,506 356,624,276    
Deferred tax liabilities, non-current 28,203,218 29,900,565    
Total liabilities 616,757,621 519,649,069    
Financial statement amounts included in the consolidated financial statements        
Total revenues 904,498,793 731,078,833 462,439,368  
Cost of revenues (306,133,210) (274,035,806) (203,170,685)  
Net income/(loss) 52,337,158 51,086,289 (71,049,371)  
Net cash provided by operating activities 23,982,431 113,892,140 (22,183,296)  
Net cash used in investing activities (112,759,658) (53,093,289) (33,425,540)  
Net cash provided by (used in) financing activities 307,159,706 135,756,603 (125,773,431)  
Consolidated VIE's assets that are collateral for the VIEs' obligations 0      
Group's VIEs        
Financial statement balances included in the consolidated financial statements        
Cash and cash equivalents 99,718,317 71,095,466    
Accounts receivable, net of allowance for doubtful accounts 118,223,577 87,835,551    
Other current assets 34,132,543 29,693,275    
Amounts due from related parties 424,864      
Total current assets 252,499,301 188,624,292    
Total non-current assets 55,033,244 49,517,785    
TOTAL ASSETS 307,532,545 238,142,077    
Accounts payable 600,735 1,505,942    
Accrued payroll and welfare expenses 44,321,824 29,309,329    
Income tax payable 28,337,431 28,793,459    
Other tax payable 16,032,365 11,188,055    
Amounts due to related parties 4,175,247 2,383,293    
Advance from customers and deferred revenue 5,073,492 7,150,344    
Liability for exclusive rights, current   8,967,972    
Other current liabilities 36,291,161 9,917,349    
Total current liabilities 134,832,255 99,215,743    
Deferred tax liabilities, non-current 469,579 655,563    
Total liabilities 135,301,834 99,871,306    
Financial statement amounts included in the consolidated financial statements        
Total revenues 492,253,803 321,004,846 172,402,066  
Cost of revenues (43,760,890) (59,920,429) (54,276,512)  
Net income/(loss) (8,699,386) 1,503,897 (3,212,138)  
Net cash provided by operating activities 55,495,458 72,877,862 16,020,624  
Net cash used in investing activities (17,245,460) (18,042,241) (17,544,270)  
Net cash provided by (used in) financing activities $ (17,043,942) $ (40,248,296) $ 26,686,813