Significant Accounting Policies - Breakdown of Revenues Between Fixed and Variable (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue Recognition
Financial Designation, Predecessor and Successor [Fixed List]			Predecessor		Predecessor
Gross revenues	$ 200,981		$ 165,253	$ 578,258	$ 505,771
Transaction fees
Revenue Recognition
Gross revenues	112,746		92,582	319,338	273,751
Commissions
Revenue Recognition
Gross revenues	37,590		24,394	108,200	79,830
Other
Revenue Recognition
Gross revenues	2,007		2,587	6,070	8,209
Variable
Revenue Recognition
Gross revenues	115,941		83,515	326,873	258,761
Variable | Transaction fees
Revenue Recognition
Gross revenues	87,574		68,855	246,066	208,049
Variable | Subscription Fees including Refinitiv market data
Revenue Recognition
Gross revenues	440		405	1,330	1,305
Variable | Commissions
Revenue Recognition
Gross revenues	27,840		14,241	78,785	49,367
Variable | Other
Revenue Recognition
Gross revenues	87		$ 14	692	$ 40
Fixed
Revenue Recognition
Financial Designation, Predecessor and Successor [Fixed List]			Predecessor		Predecessor
Gross revenues	85,040		$ 81,738	251,385	$ 247,010
Fixed | Transaction fees
Revenue Recognition
Gross revenues	25,172		23,727	73,272	65,702
Fixed | Subscription Fees including Refinitiv market data
Revenue Recognition
Gross revenues	48,198		45,285	143,320	142,676
Fixed | Commissions
Revenue Recognition
Gross revenues	9,750		10,153	29,415	30,463
Fixed | Other
Revenue Recognition
Gross revenues	$ 1,920		$ 2,573	$ 5,378	$ 8,169
Special Mention
Revenue Recognition
Allocation term		3 years