Income Taxes - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount									$ 435
Federal income tax rate after enactment of Tax Reform (as a percent)									21.00%
Provisional tax benefit, amount										$ 555
Liabilities recorded for interest related to uncertain tax positions	$ 21				$ 0				$ 21	0
Unrecognized tax benefits that would impact effective tax rate					0					0
Current:
Federal and foreign									(39)	777	$ 686
State and local									31	130	115
Total current									(8)	907	801
Deferred:
Federal and foreign									408	(736)	(103)
State and local									94	(37)	(20)
Total deferred									502	(773)	(123)
Income tax expense (benefit)	172	$ 111	$ 81	$ 130	$ (378)	$ 168	$ 170	$ 174	$ 494	$ 134	$ 678
Effective income tax rate:
Federal statutory income tax rate (as a percent)					35.00%				21.00%	35.00%	35.00%
State income taxes-net of federal effect (as a percent)									6.10%	3.40%	3.50%
Tax Reform (as a percent)									(0.00%)	(31.00%)	(0.00%)
Accounting method changes (as a percent)									(3.90%)	(0.00%)	(0.00%)
Other (as a percent)									(0.30%)	0.10%	0.00%
Effective income tax rate (as a percent)									22.90%	7.50%	38.50%
Deferred tax liabilities:
Property, plant and equipment	(1,026)				$ (933)				$ (1,026)	$ (933)
Intangibles assets	(419)				(553)				(419)	(553)
Total deferred tax liabilities	(1,445)				(1,486)				(1,445)	(1,486)
Deferred tax assets:
Payable to affiliate due to post-retirement benefit plan participation	297				366				297	366
Other	58				127				58	127
Gross deferred tax assets	355				493				355	493
Less valuation allowance on deferred tax assets	(8)				(8)				(8)	(8)
Net deferred tax assets	347				485				347	485
Net deferred tax liabilities	$ (1,098)				$ (1,001)				$ (1,098)	$ (1,001)