Income Taxes, Reconciliation (Details) - USD ($) $ in Millions			3 Months Ended		4 Months Ended	6 Months Ended
		Mar. 05, 2020	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reconciliation of the benefit from (provision for) income taxes [Abstract]
Pre-Tax Net Book Income ("NBI")			$ 1,177.2	$ 1,120.5		$ 2,549.8	$ 2,316.3
Texas Margin Tax	[1]		(24.1)	(7.0)		(27.4)	(14.7)
State income tax benefit (provision), net of federal benefit	[2]		(0.3)	(3.2)		(0.9)	8.1
Federal income tax benefit (provision) computed by applying the federal statutory rate to NBI of corporate entities			(3.4)	(49.5)		(6.5)	58.3
Federal benefit attributable to settlement of Liquidity Option Agreement	[2]		0.0	0.0		0.0	67.8
Valuation allowance on deferred tax assets	[3]		(3.4)	0.0		(6.2)	0.0
Other			0.0	0.0		(0.2)	0.0
Total benefit from (provision for) income taxes			$ (31.2)	$ (59.7)		$ (41.2)	$ 119.5
Effective income tax rate			(2.70%)	(5.30%)		(1.60%)	5.20%
OTA Holdings, Inc. [Member]
Reconciliation of the benefit from (provision for) income taxes [Abstract]
Total benefit from (provision for) income taxes		$ 72.2	$ (7.0)	$ (50.5)	$ 64.5	$ (13.3)	$ 136.7

[1] Although the Texas Margin Tax is not considered a state income tax, it has the characteristics of an income tax since it is determined by applying a tax rate to a base that considers our Texas-sourced revenues and expenses.

[2] The total benefit recognized in income tax expense on March 5, 2020 from settlement of the Liquidity Option was $72.2 million, which is comprised of $4.4 million of state income tax benefit and $67.8 million of federal income tax benefit.

[3] Management believes that it is more likely than not that the net deferred tax assets attributable to OTA will not be fully realizable; therefore, we have provided for a valuation allowance.