Fair Value Measurements - Summary of Categorization of Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Liabilities:
Deferred consideration	$ 157,147	$ 161,540
Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	566	24
Securities owned, at fair value	6,419	8,873
Total	6,985	8,897
Liabilities:
Deferred consideration	157,147	161,540
Securities sold, but not yet purchased	1,338	1,698
Total	158,485	163,238
Fair Value, Measurements, Nonrecurring [Member]
Assets:
Thesys—Series Y preferred stock		3,080
Total		3,080
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	566	24
Securities owned, at fair value	6,419	8,873
Total	6,985	8,897
Liabilities:
Securities sold, but not yet purchased	1,338	1,698
Total	1,338	1,698
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities:
Deferred consideration	157,147	161,540
Total	$ 157,147	161,540
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Assets:
Thesys—Series Y preferred stock		3,080
Total		$ 3,080