Summary of Significant Accounting Policies - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Common Stock Warrant Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	$ 512	$ 809
Contingent Purchase Consideration [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	76,900	52,800
Cash Equivalents [Member]
Assets
Assets measured at fair value on a recurring basis	289,782	87,792
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Common Stock Warrant Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Contingent Purchase Consideration [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash Equivalents [Member]
Assets
Assets measured at fair value on a recurring basis	289,782	87,792
Significant Other Observable Inputs (Level 2) [Member] | Common Stock Warrant Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) [Member] | Contingent Purchase Consideration [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	0	0
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents [Member]
Assets
Assets measured at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock Warrant Liabilities [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	512	809
Significant Unobservable Inputs (Level 3) [Member] | Contingent Purchase Consideration [Member]
Liabilities
Liabilities measured at fair value on a recurring basis	76,900	52,800
Significant Unobservable Inputs (Level 3) [Member] | Cash Equivalents [Member]
Assets
Assets measured at fair value on a recurring basis	$ 0	$ 0