Condensed Financial Information of the Parent Company - Condensed Balance Sheets (Detail)
¥ in Thousands, $ in Thousands
Dec. 31, 2017
CNY (¥)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
CNY (¥)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
CNY (¥)
Dec. 31, 2014
CNY (¥)
Current assets:            
Cash and cash equivalents ¥ 911,588 $ 140,109 ¥ 3,293,911 $ 506,265 ¥ 2,152,647 ¥ 1,054,416
Short-term investments 7,242,636 1,113,173 2,430,091      
Prepaid expenses and other current assets 186,123 28,608 377,219      
Total current assets 10,258,586 1,576,718 7,432,532      
Non-current assets:            
Other non-current assets 17,361 2,668 34,037      
Total non-current assets 2,036,389 312,987 1,959,494      
Total assets 12,294,975 1,889,705 9,392,026      
Current liabilities:            
Accrued expenses and other payables 1,658,934 254,973 1,151,547      
Dividend payable 595,779 91,570 0      
Total current liabilities 3,889,316 597,778 2,544,040      
Total liabilities 4,359,689 670,073 3,040,813      
Commitments and Contingencies      
Shareholders' equity:            
Ordinary shares 7,909 1,216 7,784      
Additional paid-in capital 3,246,475 498,974 3,006,152      
Accumulated other comprehensive income 69,954 10,752 125,009      
Retained earnings 4,627,299 711,203 3,221,459      
Total shareholders' equity 7,951,637 1,222,145 6,360,404      
Total liabilities and equity 12,294,975 1,889,705 9,392,026      
Parent Company [Member]            
Current assets:            
Cash and cash equivalents 31,781 4,885 57,505 $ 8,838 ¥ 136,810 ¥ 234,034
Short-term investments 699,159 107,459 486,341      
Prepaid expenses and other current assets 23,315 3,583 26,113      
Total current assets 754,255 115,927 569,959      
Non-current assets:            
Other non-current assets 3,816 587 5,936      
Investment in subsidiaries and VIEs 7,798,858 1,198,663 5,807,841      
Total non-current assets 7,802,674 1,199,250 5,813,777      
Total assets 8,556,929 1,315,177 6,383,736      
Current liabilities:            
Accrued expenses and other payables 6,932 1,065 20,660      
Dividend payable 595,779 91,570 0      
Due to subsidiaries 2,581 397 2,672      
Total current liabilities 605,292 93,032 23,332      
Total liabilities 605,292 93,032 23,332      
Commitments and Contingencies      
Shareholders' equity:            
Ordinary shares 7,909 1,216 7,784      
Additional paid-in capital 3,246,475 498,974 3,006,152      
Accumulated other comprehensive income 69,954 10,752 125,009      
Retained earnings 4,627,299 711,203 3,221,459      
Total shareholders' equity 7,951,637 1,222,145 6,360,404      
Total liabilities and equity ¥ 8,556,929 $ 1,315,177 ¥ 6,383,736