Basis of Presentation - Schedule of Aforementioned Revisions to Consolidated Statements of Financial Condition (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Error Corrections And Prior Period Adjustments Restatement [Line Items]
Goodwill	$ 127,536	$ 127,536	$ 54,562	$ 51,975	$ 51,975	$ 51,975
Deferred tax assets, net	42,287		47,376
Total assets	$ 4,917,409		3,366,130
As Reported
Error Corrections And Prior Period Adjustments Restatement [Line Items]
Goodwill			51,975
Deferred tax assets, net			49,963
Total assets			3,366,130
Adjustment
Error Corrections And Prior Period Adjustments Restatement [Line Items]
Goodwill			2,587
Deferred tax assets, net			$ (2,587)