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Significant Accounting Policies (Schedule Of Securities Segregated For Regulatory Purposes) (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Marketable Securities [Line Items] |  |  |
Securities - segregated for regulatory purposes |  | $ 14,400 | $ 27,821
US Government Securities [Member] |  |  |
Marketable Securities [Line Items] |  |  |
Securities - segregated for regulatory purposes |  | 3,925 | 4,750
Securities Purchased Under Agreement To Resell [Member] |  |  |
Marketable Securities [Line Items] |  |  |
Securities - segregated for regulatory purposes | [1] | $ 10,475 | $ 23,071
[1] | These balances are collateralized by U.S. government securities.

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