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Related Party Transactions - Consolidated Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transactions
Financial Designation, Predecessor and Successor [Fixed List]	Successor	Successor	Successor	Predecessor	Successor	Successor	Predecessor
Revenue:
Revenue		$ 178,637		$ 478,941	$ 775,566		$ 504,448
Shared Services Fees:
Technology and communications		9,907		26,598	39,285		30,013
General and administrative		11,837		23,056	34,960		33,973
Occupancy		$ 3,308		$ 10,732	$ 14,686		$ 14,441
Affiliated Entity
Related Party Transactions
Financial Designation, Predecessor and Successor [Fixed List]		Successor		Predecessor	Successor		Predecessor
Operating Income:
Net interest income (expense)		$ 17		$ 34	$ 858		$ (415)
Shared Services Fees:
Technology and communications		740		2,220	2,960		2,960
General and administrative		180		539	430		719
Occupancy		155		466	481		621
Transaction fees \| Affiliated Entity
Revenue:
Revenue		59,259		159,663	59,643		151,695
Subscription fees \| Affiliated Entity
Revenue:
Revenue		5,718		16,627	5,670		37,426
Commissions \| Affiliated Entity
Revenue:
Revenue		12,401		34,944	16,186		43,315
Refinitiv market data fees \| Affiliated Entity
Revenue:
Revenue		$ 13,467		$ 36,851	$ 55,635		$ 50,125