Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency [Line Items]
Liabilities recorded for interest related to uncertain tax positions					$ 0					$ 0	$ 0
Liabilities recorded for penalties related to uncertain tax positions					0					0	0
Current tax provision:
Federal and foreign									$ 734	738	653
State and local									114	129	101
Total current tax provision									848	867	754
Deferred tax expense:
Federal and foreign									(170)	(209)	(125)
State and local									(19)	(19)	(27)
Total deferred tax expense									(189)	(228)	(152)
Income tax expense	$ 169	$ 171	$ 152	$ 167	161	$ 156	$ 162	$ 160	$ 659	$ 639	$ 602
Effective income tax rate:
Federal statutory income tax rate (as a percent)									35.00%	35.00%	35.00%
State income taxes-net of federal effect (as a percent)									3.60%	4.00%	3.10%
Other (as a percent)									(0.60%)	0.70%	0.30%
Effective income tax rate (as a percent)									38.00%	39.70%	38.40%
Deferred tax liabilities:
Property, plant and equipment	(1,431)				(1,380)				$ (1,431)	$ (1,380)
Intangibles assets	(1,153)				(1,449)				(1,153)	(1,449)
Receivable from an affiliate due to pension plan participation	(460)				(500)				(460)	(500)
Other	(59)				(52)				(59)	(52)
Total deferred tax liabilities	(3,103)				(3,381)				(3,103)	(3,381)
Deferred tax assets:
Payable to affiliate due to post-retirement plan participation	921				998				921	998
Debt premiums	21				36				21	36
Other	277				274				277	274
Total deferred tax assets	1,219				1,308				1,219	1,308
Valuation allowance on deferred tax assets	(12)				(12)				(12)	(12)
Net deferred tax assets	1,207				1,296				1,207	1,296
Net deferred tax liabilities	(1,896)				$ (2,085)				(1,896)	$ (2,085)
Domestic tax authority
Income Tax Contingency [Line Items]
Liabilities recorded for interest related to uncertain tax positions	0								0
Liabilities recorded for penalties related to uncertain tax positions	$ 0								$ 0