Income Taxes, Reconciliation (Details) - USD ($) $ in Millions			1 Months Ended	3 Months Ended
		Mar. 05, 2020	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Reconciliation of the benefit from (provision for) income taxes [Abstract]
Pre-Tax Net Book Income ("NBI")				$ 1,372.6	$ 1,195.8
Texas Margin Tax	[1]			(3.3)	(7.7)
State income tax benefit (provision), net of federal benefit	[2]			(0.6)	11.3
Federal income tax benefit (provision) computed by applying the federal statutory rate to NBI of corporate entities				(3.1)	107.8
Federal benefit attributable to settlement of Liquidity Option Agreement	[2]			0.0	67.8
Valuation allowance on deferred tax assets	[3]			(2.8)	0.0
Other				(0.2)	0.0
Total benefit from (provision for) income taxes				$ (10.0)	$ 179.2
Effective income tax rate				(0.70%)	15.00%
OTA Holdings, Inc. [Member]
Reconciliation of the benefit from (provision for) income taxes [Abstract]
Total benefit from (provision for) income taxes		$ 72.2	$ 115.0	$ (6.3)	$ 187.2

[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 in March 2020 from settlement of the Liquidity Option Agreement 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.  Accordingly, we provided for a valuation allowance against OTA’s net deferred tax assets at March 31, 2021.