ITG Acquisition - Intangible Assets (Details) - USD ($) $ in Thousands	Mar. 01, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets		$ 513,153		$ 83,989
Goodwill		1,195,477		836,583
Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets		400,688		$ 43,095
Trade names
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets		2,900
ITG
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 479,600
Goodwill	358,955	$ 358,955	$ 360,434
Total	838,555
ITG | Execution Services
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Goodwill	359,000
ITG | Technology
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 93,000
Amortization Years	5 years
ITG | Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 383,000
Amortization Years	10 years
ITG | Trade names
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 3,600
Amortization Years	3 years