Income Taxes - Tax Carryforwards and Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	$ 299	$ 380
Tax credit carryforwards	3,859	$ 3,406
Federal [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	2
Tax credit carryforwards	3,508
State [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	280
Tax credit carryforwards	348
Tax credit carryforward with indefinite expiration period	191
Foreign [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	17
Tax credit carryforwards	$ 3