Investment in Unconsolidated Affiliate and Related Party Transactions Reconciliation of Basis Difference (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Basis Difference [Line Items]
Amortization of basis difference	[1]	$ 22.1	$ 28.4	$ 28.5
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		652.5	$ 680.3
OGE Energy [Member]
Reconciliation of Basis Difference [Line Items]
Amortization of basis difference	[2]	$ 27.8

[1] Includes loss on dilution, net of proportional basis difference recognition.

[2] Includes proportional basis difference recognition due to dilution.