Summary of Significant Accounting Policies - Reconciliation of Liabilities Measured at Fair Value Using Significant Observable Inputs (Level 3) (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Contingent Purchase Consideration [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 52,800	$ 51,000
Additions	0	0
Settlements	0	0
Changes in fair value	24,100	1,800
Ending Balance	76,900	52,800
Common Stock Warrant Liability [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	809	6,196
Additions	0	0
Settlements	0	0
Changes in fair value	(297)	(5,387)
Ending Balance	$ 512	$ 809