Related Party Transactions - Balances (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
Cash and cash equivalents		$ 283,790				$ 283,790
Receivables from brokers and dealers and clearing organizations		3,332				3,332
Deposits with clearing organizations		500				500
Accounts receivable		40,730				40,730
Receivable from affiliates		3,243	$ 2,525		$ 2,525	3,243
Other assets		9				9
Payable to brokers and dealers and clearing organizations		2,404				2,404
Deferred revenue		9,151	4,733		4,733	9,151
Payable to affiliates		$ 5,009	$ 1,506		$ 1,506	$ 5,009