Condensed Financial Information of the Parent Company - Condensed Statements of Cash Flows (Detail)
¥ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2017
CNY (¥)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
CNY (¥)
Dec. 31, 2015
CNY (¥)
Net cash used in operating activities ¥ 2,473,018 $ 380,096 ¥ 1,625,896 ¥ 1,461,841
Net cash used in investing activities (4,913,827) (755,241) (514,906) (407,657)
Net cash generated from financing activities 61,070 9,386 27,920 30,200
Effect of exchange rate changes on cash and cash equivalents (2,584) (397) 2,354 13,847
Net decrease in cash and cash equivalents (2,382,323) (366,156) 1,141,264 1,098,231
Cash and cash equivalents at beginning of year 3,293,911 506,265 2,152,647 1,054,416
Cash and cash equivalents at end of year 911,588 140,109 3,293,911 2,152,647
Parent Company [Member]        
Net cash used in operating activities (8,303) (1,276) (23,846) (2,017)
Net cash used in investing activities (77,005) (11,835) (86,743) (132,976)
Net cash generated from financing activities 61,070 9,386 25,420 30,200
Effect of exchange rate changes on cash and cash equivalents (1,486) (228) 5,864 7,569
Net decrease in cash and cash equivalents (25,724) (3,953) (79,305) (97,224)
Cash and cash equivalents at beginning of year 57,505 8,838 136,810 234,034
Cash and cash equivalents at end of year ¥ 31,781 $ 4,885 ¥ 57,505 ¥ 136,810