Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended							9 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2011	Dec. 31, 2012	Mar. 31, 2011 Predecessor	Dec. 31, 2010 Predecessor
Additional information
Liabilities recorded for interest related to uncertain tax positions	$ 5				$ 5			$ 5	$ 5
Current tax provision (benefit):
Federal								173	638	104	470
State and local								26	105	11	80
Total current tax (benefit) provision								199	743	115	550
Deferred tax expense (benefit)
Federal								128	(175)	61	208
State and local								22	(26)	15	33
Total deferred tax expense								150	(201)	76	241
Total deferred tax expense (benefit)	160	133	113	136	115	118	116	349	542	191	791
Effective income tax rate:
Federal statutory income tax rate (as a percent)								35.00%	35.00%	35.00%	35.00%
State income taxes-net of federal effect (as a percent)								3.50%	3.70%	3.40%	3.90%
Medicare subsidiary (as a percent)											2.70%
Excess compensation (as a percent)											1.00%
Other (as a percent)								0.60%	0.30%	0.60%	(0.40%)
Effective income tax rate (as a percent)								39.10%	39.00%	39.00%	42.20%
Deferred tax liabilities:
Property, plant and equipment	(1,046)				(1,279)			(1,279)	(1,046)
Intangibles assets	(2,226)				(2,274)			(2,274)	(2,226)
Receivable from an affiliate due to pension plan participation	(398)				(359)			(359)	(398)
Other	(39)				(148)			(148)	(39)
Total deferred tax liabilities	(3,709)				(4,060)			(4,060)	(3,709)
Deferred tax assets:
Payable to affiliate due to post-retirement benefit plan participation	932				920			920	932
Debt premiums	70				164			164	70
Other	239				304			304	239
Total deferred tax asset	1,241				1,388			1,388	1,241
Valuation allowance on deferred tax assets	(12)				(8)			(8)	(12)
Net deferred tax assets	1,229				1,380			1,380	1,229
Net deferred tax liabilities	$ (2,480)				$ (2,680)			$ (2,680)	$ (2,480)