Summary of Significant Accounting Policies Asset Retirement Obligation (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1		$ 73.5	$ 83.9
Accretion expense		0.5	1.0
Revisions in estimated cash flows	[1]	5.8	(2.4)
Liabilities settled	[2]	(0.2)	(9.0)
Balance at December 31		$ 79.6	$ 73.5

[1] Assumptions changed related to the estimated timing and estimated cost of the removal of asbestos at OG&E's generating facilities.

[2] Asset retirement obligations were settled for asbestos removal at one of OG&E's generating facilities