Benefits - Net Periodic Benefit Cost (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	3 Months Ended								6 Months Ended	9 Months Ended	6 Months Ended		9 Months Ended		6 Months Ended		9 Months Ended		3 Months Ended
	Sep. 30, 2012	Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member]		Sep. 30, 2012 Other Postretirement Benefit Plans, Defined Benefit [Member]		Sep. 30, 2013 Pension Plans, Defined Benefit [Member]		Sep. 30, 2012 Pension Plans, Defined Benefit [Member]		Sep. 30, 2012 Successor [Member]	Sep. 30, 2013 Successor [Member]	Sep. 30, 2012 Successor [Member] Other Postretirement Benefit Plans, Defined Benefit [Member]		Sep. 30, 2013 Successor [Member] Other Postretirement Benefit Plans, Defined Benefit [Member]		Sep. 30, 2012 Successor [Member] Pension Plans, Defined Benefit [Member]		Sep. 30, 2013 Successor [Member] Pension Plans, Defined Benefit [Member]		Mar. 25, 2012 Predecessor [Member]	Mar. 25, 2012 Predecessor [Member] Other Postretirement Benefit Plans, Defined Benefit [Member]		Mar. 25, 2012 Predecessor [Member] Pension Plans, Defined Benefit [Member]
Net Periodic Benefit Cost:
Service Cost		$ (1)		$ 0		$ 0		$ 1				$ 0		$ 0		$ 2		$ 3			$ 1		$ 1
Interest Cost		1		1		2		2				1		2		5		6			1		2
Expected return on plan assets		(2)		(2)		(3)		(3)				(3)		(5)		(6)		(9)			(1)		(2)
Prior service cost (credit) amortization		1		0		0		0				0		1		0		0			(1)		0
Actuarial loss (gain) amortization		0		0		1		0				0		0		0		2			0		2
Curtailment recognition	(15)									(15)	0	(15)	[1]	0	[1]	0	[1]	0	[1]	0	0	[1]	0	[1]
Net periodic benefit cost subtotal		(1)		(1)		0		0				(17)		(2)		1		2			0		3
Regulatory adjustment		0	[2]	1	[2]	1	[2]	3	[2]			1	[2]	0	[2]	6	[2]	5	[2]		1	[2]	0	[2]
Net periodic benefit cost		$ (1)		$ 0		$ 1		$ 3				$ (16)		$ (2)		$ 7		$ 7			$ 1		$ 3

[1] Subsequent to the ETE Merger, the Company amended certain of its OPEB plans, which prospectively restrict participation in the plans for the impacted active employees. The plan amendments resulted in the plans becoming currently over-funded and, accordingly, the Company recorded a pre-tax curtailment gain of $75 million. Such gain was offset by establishment of a non-current refund liability in the amount of $60 million. As such, the net curtailment gain recognition was $15 million.

[2] The Company has historically recovered certain qualified pension benefit plan and other postretirement benefit plan costs through rates charged to utility customers in its MGE and NEG divisions. Certain utility commissions require that the recovery of these costs be based on the Employee Retirement Income Security Act of 1974, as amended, or other utility commission specific guidelines. The difference between these regulatory-based amounts and the periodic benefit cost calculated pursuant to GAAP is deferred as a regulatory asset or liability and amortized to expense over periods in which this difference will be recovered in rates, as promulgated by the applicable utility commission.