Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Provisional tax benefit, amount	$ 555
Current:
Federal and foreign									$ 777	$ 686	$ 734
State and local									130	115	114
Total current									907	801	848
Deferred:
Federal and foreign									(736)	(103)	(170)
State and local									(37)	(20)	(19)
Total deferred									(773)	(123)	(189)
Income tax expense	$ (378)	$ 168	$ 170	$ 174	$ 152	$ 159	$ 179	$ 188	$ 134	$ 678	$ 659
Effective income tax rate:
Federal statutory income tax rate (as a percent)	35.00%								35.00%	35.00%	35.00%
Federal income tax rate after enactment of Tax Reform (as a percent)	21.00%
State income taxes-net of federal effect (as a percent)									3.40%	3.50%	3.60%
Tax Reform, Percent									(31.00%)	(0.00%)	(0.00%)
Other (as a percent)									0.10%	0.00%	(0.60%)
Effective income tax rate (as a percent)									7.50%	38.50%	38.00%
Deferred tax liabilities:
Property, plant and equipment	$ (933)				(1,384)				$ (933)	$ (1,384)
Intangibles assets	(553)				(1,088)				(553)	(1,088)
Receivable from an affiliate due to pension plan participation	0				(452)				0	(452)
Total deferred tax liabilities	(1,486)				(2,924)				(1,486)	(2,924)
Deferred tax assets:
Payable to affiliate due to post-retirement plan participation	366				954				366	954
Other	127				209				127	209
Gross deferred tax assets	493				1,163				493	1,163
Less valuation allowance on deferred tax assets	(8)				(12)				(8)	(12)
Net deferred tax assets	485				1,151				485	1,151
Net deferred tax liabilities	(1,001)				(1,773)				(1,001)	(1,773)
Change in valuation allowance of deferred tax assets									4
Domestic tax authority
Income Tax Contingency [Line Items]
Liabilities recorded for interest related to uncertain tax positions	0				0				0	0	$ 0
Liabilities recorded for penalties related to uncertain tax positions	$ 0				$ 0				$ 0	$ 0	$ 0