Income Taxes, Reconciliation (Details) - USD ($) $ in Millions			3 Months Ended		7 Months Ended	9 Months Ended
		Mar. 05, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Reconciliation of the provision for (benefit from) income taxes [Abstract]
Pre-Tax Net Book Income ("NBI")			$ 1,064.9	$ 1,060.2		$ 3,381.2	$ 3,599.1
Texas Margin Tax	[1]		(7.2)	(15.5)		(21.9)	(36.5)
State income tax benefit (provision), net of federal benefit	[2]		1.6	0.0		9.7	(0.3)
Federal income tax benefit (provision) computed by applying the federal statutory rate to NBI of corporate entities			25.1	0.1		83.4	(0.6)
Federal benefit attributable to settlement of Liquidity Option	[2]		0.0	0.0		67.8	0.0
Other permanent differences			(0.4)	0.0		(0.4)	0.0
Total benefit from (provision for) income taxes			$ 19.1	$ (15.4)		$ 138.6	$ (37.4)
Effective income tax rate			1.80%	(1.50%)		4.10%	(1.00%)
OTA Holdings, Inc. [Member]
Reconciliation of the provision for (benefit from) income taxes [Abstract]
Total benefit from (provision for) income taxes		$ 72.2	$ 21.3		$ 85.8	$ 158.0

[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.