Income Taxes - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Taxes Other Information [Line Items]
Income Tax Expense (Benefit)	$ 95		$ 225	$ 412	$ 606
Effective Income Tax Rate Reconciliation, Percent	11.00%		29.00%	16.00%	28.00%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				21.00%
Other Tax Expense (Benefit)				$ (71)	$ 26
Excess Tax Benefit from Share-based Compensation, Financing Activities				(22)
Unrecognized Tax Benefits Related to the Expiration of the Applicable Statute of Limitations				(8)
Adjustment to Deferred Taxes Remeasurement Benefit Related to 2017 Tax Act				(55)
Tax Expense (Benefit) from Valuation Allowance Adjustment	$ 21	$ 35		14
Deferred Tax Assets, Valuation Allowance	$ 89			$ 89		$ 143