Income Taxes, Reconciliation of the Benefit From (Provision For) Income Taxes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of the benefit from (provision for) income taxes [Abstract]
Pre-Tax Net Book Income ("NBI")		$ 3,761.4	$ 4,732.7	$ 4,298.8
Texas Margin Tax	[1]	(32.1)	(44.2)	(54.8)
State income tax benefit (provision), net of federal benefit	[2]	9.2	(0.5)	(0.2)
Federal income tax benefit (provision) computed by applying the federal statutory rate to NBI of corporate entities		80.1	(0.9)	(2.1)
Federal benefit attributable to settlement of Liquidity Option	[2]	67.8	0.0	0.0
Other differences		(0.7)	0.0	(3.2)
Total benefit from (provision for) income taxes		$ 124.3	$ (45.6)	$ (60.3)
Effective income tax rate		3.30%	(1.00%)	(1.40%)

[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.