Fair Value of Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Restricted cash	$ 20	$ 10
Liabilities:
Contingent consideration	49,500	39,600
Unobservable Inputs (Level 3)
Liabilities:
Contingent consideration	49,500	39,600
Recurring
Assets:
Cash and cash equivalents	57,856	14,680
Restricted cash	20	10
Total assets	57,876	14,690
Liabilities:
Contingent consideration	49,500	39,600
Total liabilities	49,500	39,600
Recurring | Active Markets (Level 1)
Assets:
Cash and cash equivalents	57,856	14,680
Restricted cash	20	10
Total assets	57,876	14,690
Liabilities:
Contingent consideration	0	0
Total liabilities	0	0
Recurring | Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Total assets	0	0
Liabilities:
Contingent consideration	0	0
Total liabilities	0	0
Recurring | Unobservable Inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Total assets	0	0
Liabilities:
Contingent consideration	49,500	39,600
Total liabilities	$ 49,500	$ 39,600