Acquisitions and Dispositions (Details) $ in Millions		1 Months Ended		3 Months Ended				9 Months Ended	10 Months Ended
		May 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) MW	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Jul. 31, 2022 USD ($)	Jul. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Obligations Under Operating Leases				$ 586.8				$ 586.8			$ 638.4
Asset Retirement Obligation, Liabilities Settled	[1],[2],[3],[4]							27.6
Additional Paid in Capital				7,075.9				7,075.9			6,588.9
Property, Plant and Equipment				24,135.9				24,135.9			23,133.9
Noncontrolling Interests				$ 249.1				$ 249.1			223.6
Dry Lake Solar Project
Noncash or Part Noncash Acquisition, Other Assets Acquired						$ 18.9
Dry Lake Solar Project | Noncontrolling Interest [Member]
Noncash or Part Noncash Acquisition, Other Assets Acquired						18.9
Generation And Marketing [Member] | Conesville Generating Station [Member]
Payments for (Proceeds from) Removal Costs			$ 26.0				$ 98.0		$ 34.0	$ 38.0
Asset Retirement Obligation, Liabilities Settled							106.0
Generation And Marketing [Member] | Dry Lake Solar Project
Percentage of an Asset Acquired				75.00%				75.00%
Wind Generation MWs | MW				100
Noncash or Part Noncash Acquisition, Value of Assets Acquired				$ 114.0
Payments to Acquire Businesses, Gross		$ 11.0				$ 103.0
Property, Plant and Equipment				146.0				$ 146.0
Noncontrolling Interests				$ 35.0				35.0
Generation And Marketing [Member] | Dry Lake Solar Project | Noncontrolling Interest [Member]
Noncash or Part Noncash Acquisition, Other Assets Acquired								16.0
Generation And Marketing [Member] | Trent and Desert Sky Wind Farms [Member]
Payments to Acquire Businesses, Gross							57.0
Redeemable Noncontrolling Interest							63.0
Additional Paid in Capital							$ 6.0
Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities
Wind Generation MWs | MW				1,485
Noncash or Part Noncash Acquisition, Value of Assets Acquired				$ 2,000.0
Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities | Sundance [Member]
Wind Generation MWs | MW				199
Payments to Acquire Businesses, Gross					$ 270.0
Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities | Maverick [Member]
Wind Generation MWs | MW				287
Payments to Acquire Businesses, Gross				$ 383.0
Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities | Traverse [Member]
Wind Generation MWs | MW				999
Public Service Co Of Oklahoma [Member]
Obligations Under Operating Leases				$ 63.7				63.7			36.2
Additional Paid in Capital				1,039.0				1,039.0			414.0
Property, Plant and Equipment				$ 1,795.2				$ 1,795.2			1,480.7
Public Service Co Of Oklahoma [Member] | Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities
Percentage of an Asset Acquired				45.50%				45.50%
Property, Plant and Equipment				$ 314.0				$ 314.0
Public Service Co Of Oklahoma [Member] | Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities | Sundance [Member]
Obligations Under Operating Leases				13.0				13.0
Public Service Co Of Oklahoma [Member] | Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities | Maverick [Member]
Obligations Under Operating Leases				18.0				18.0
Southwestern Electric Power Co [Member]
Obligations Under Operating Leases				79.2				79.2			44.1
Additional Paid in Capital				1,092.2				1,092.2			812.2
Property, Plant and Equipment				5,065.5				5,065.5			4,681.4
Noncontrolling Interests				$ 0.4				$ 0.4			$ 1.6
Southwestern Electric Power Co [Member] | Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities
Percentage of an Asset Acquired				54.50%				54.50%
Property, Plant and Equipment				$ 376.0				$ 376.0
Southwestern Electric Power Co [Member] | Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities | Sundance [Member]
Obligations Under Operating Leases				15.0				15.0
Southwestern Electric Power Co [Member] | Vertically Integrated Utilities [Member] | North Central Wind Energy Facilities | Maverick [Member]
Obligations Under Operating Leases				$ 22.0				$ 22.0

[1] Includes ARO related to Sabine and DHLC.

[2] Includes ARO related to asbestos removal.

[3] Includes ARO related to ash disposal facilities.

[4] Includes ARO related to nuclear decommissioning costs for the Cook Plant of $1.85 billion and $1.80 billion as of September 30, 2021 and December 31, 2020, respectively.