Income Taxes - Tax Carryforwards and Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	$ 482	$ 427
Tax credit carryforwards	3,126	$ 3,059
Federal [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	158
Tax credit carryforwards	2,779
State [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	232
Tax credit carryforwards	347
Tax credit carryforward with indefinite expiration period	188
Foreign [Member]
Operating Loss and Tax Credit Carryforwards [Line Items]
Net operating loss carryforwards	92
Operating loss carryforwards with indefinite expiration period	$ 64