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Description of Business and Accounting Policies - Fair Value of Financial Instruments (Details) - Equity Warrant - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Derivative gains (losses) $ 109 $ 67 $ 0
Level 2 Securities      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Fair value of warrants $ 441 $ 223