Fair Value Measurements - Summary of Categorization of Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Apr. 11, 2018	Dec. 31, 2017
Liabilities:
Deferred consideration	$ 156,055	$ 172,746
Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	5,722		$ 26,548
Securities owned, at fair value	4,426		66,294
Total	10,148		92,842
Liabilities:
Deferred consideration	156,055
Securities sold, but not yet purchased	2,018		950
Total	158,073		950
Fair Value, Measurements, Nonrecurring [Member]
Assets:
AdvisorEngine Inc. - Option	3,278		3,278
Thesys Group, Inc. - Series Y preferred stock	6,909		6,909
Total	10,187		10,187
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	5,722		26,548
Securities owned, at fair value	4,426		1,691
Total	10,148		28,239
Liabilities:
Securities sold, but not yet purchased	2,018		950
Total	2,018		950
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Securities owned, at fair value			64,603
Total			64,603
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities:
Deferred consideration	156,055
Total	156,055
Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Assets:
AdvisorEngine Inc. - Option	3,278		3,278
Thesys Group, Inc. - Series Y preferred stock	6,909		6,909
Total	$ 10,187		$ 10,187