Income Taxes - Tax Carryforwards and Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	$ 350	$ 482
Tax credit carryforwards	3,259	$ 3,126
Federal [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Tax credit carryforwards	2,915
State [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	269
Tax credit carryforwards	344
Tax credit carryforward with indefinite expiration period	188
Foreign [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	81
Operating loss carryforwards with indefinite expiration period	$ 60