Assets And Liabilities Measured At Fair Value Realized Unrealized Gain Loss (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments	$ 99	$ 175
Fair Value Option Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments	114	260
Exchangeable Senior Debentures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments	17	(84)
Indemnification Assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments	(29)
Other Financial Instruments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Realized and unrealized gains (losses) on financial instruments	$ (3)	$ (1)

- Definition

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

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