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Basis of Presentation and Significant Accounting Policies Noncontrolling Interest (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 USD ($) shares | Dec. 31, 2019 USD ($) shares | Jun. 30, 2018 USD ($) shares
Stockholders' Equity Attributable to Noncontrolling Interest | $ | $ 60 | $ 60 | $ 264
Preferred Stock, Shares Outstanding | shares | 0 | 0 |
Ambac Assurance Corporation [Member] |  |  |
Preferred Stock, Shares Outstanding | shares | 5,501 | 5,501 | 26,411
Preferred Stock, Liquidation Preference, Value | $ | $ 138 | $ 138 | $ 660
London Interbank Offered Rate (LIBOR) [Member] |  |  |
Dividend Rate on Auction Rate Preferred Shares | 200 |  |

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