Noncontrolling Interests Distributions Made to Partners (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2012
Distribution Made to Partners [Line Items]
Distributions to partners	$ 12.5	$ 12.5	$ 12.5	$ 30.0	$ 67.5
OGE Holdings [Member]
Distribution Made to Partners [Line Items]
Distributions to partners	10.2	10.2	10.1	24.4	54.9
ArcLight Group [Member]
Distribution Made to Partners [Line Items]
Distributions to partners	2.3	2.3	2.4	5.6	12.6
Atoka [Member]
Distribution Made to Partners [Line Items]
Distributions to partners					0
Atoka [Member]
Distribution Made to Partners [Line Items]
Contributions from noncontrolling interest partner					$ 1.2