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 |