ITG Acquisition - Tax Treatment of the ITG Acquisition (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 01, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Deferred tax assets	$ 247,612		$ 200,359
ITG
Business Acquisition [Line Items]
Deferred tax assets		$ 17,200
Deferred tax liabilities		$ 65,900