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Share-based Compensation - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 01, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increase (decrease) in accrued employee compensation expense	$ 25.2
Class A common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	967,526	441,920	594,536
Increase (decrease) in accrued employee compensation expense	$ 2.1	$ 12.6	$ 11.2
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee compensation expense	37.4
Unrecognized share-based compensation expense	$ 37.1	$ 43.4
Weighted average period for compensation expense expected to be recognized	1 year 10 days	2 years
RSUs \| Class A common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee compensation expense	$ 3.7	$ 66.1	$ 17.9
2015 Management Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	0	156,129	0
2015 Management Incentive Plan \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Expiration period	10 years
Employee compensation expense		$ 1.4	$ 5.8
2015 Management Incentive Plan \| RSUs and RSAs \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Assumed Plan \| Class A common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance (in shares)				2,497,028
Invetment Technology Group \| Class A common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance (in shares)				1,230,406