Income Taxes - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Taxes Other Information [Line Items]
Income Tax Expense (Benefit)	$ 171	$ 222	$ 318	$ 381
Effective Income Tax Rate Reconciliation, Percent	19.00%	30.00%	19.00%	28.00%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%
Other Tax Expense (Benefit)			$ 4	$ 23
Excess Tax Benefit from Share-based Compensation, Financing Activities			20
Adjustment to Deferred Taxes Remeasurement Benefit Related to 2017 Tax Act			19
Tax Expense (Benefit) from Valuation Allowance Adjustment			(35)
Deferred Tax Assets, Valuation Allowance	$ 180		$ 180		$ 143