Summary of Significant Accounting Policies Asset Retirement Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1		$ 75.1	$ 69.6
Accretion expense		3.4	3.1
Revisions in estimated cash flows	[1]	6.8	2.4
Liabilities settled		(1.4)	0.0
Balance at December 31		$ 83.9	$ 75.1

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